Image Optimizer

A lightweight, fast, lossless image optimizer for Windows, inspired by ImageOptim on the Mac.

Drop AVIF, GIF, JPEG, JPEG XL, PNG, SVG and WebP files (or whole folders) onto the window. Each file is optimized and replaced in place. There's nothing to configure: the defaults are safe, and a small Settings panel (gear button) is there if you want it.

You can also right-click images or folders in File Explorer and choose Optimize with Image Optimizer (on Windows 11 it's under Show more options). Selecting many files at once opens them all in one window.

Click the File, Size or Savings header to sort the list (click again to reverse it). The app follows the Windows light or dark app mode automatically, and uses your High Contrast colors when a contrast theme is on. It's in English, French, German, Italian, Japanese and Spanish, and follows the Windows display language.

Light	Dark

How it stays lossless and safe

1. The original is copied to a private temporary folder; the optimizers only ever see that copy.
2. The format's optimizer writes one or more candidates (for JPEG, both baseline and progressive).
3. Candidates that aren't strictly smaller than the original are thrown away.
4. The smallest candidate is decoded and compared pixel by pixel with the original (Windows imaging for PNG/JPEG, dwebp for WebP, avifdec for AVIF, djxl for JPEG XL, and every frame of a GIF as browsers draw it, along with its timing and loop count). Any difference and it's discarded.
5. If the file changed on disk in the meantime, nothing is written.
6. Only then is the original swapped for the candidate in a single File.Replace step.

If anything fails along the way, the original file is left exactly as it was.

Format	Tool	What it does
AVIF	avifenc / avifdec from libavif	Lossless AVIFs are re-encoded losslessly at a higher effort (typically a few percent smaller). Lossy AVIFs are left untouched.
GIF	Gifsicle	-O3 lossless frame and LZW optimization. Removing metadata also drops other programs' data, such as XMP, unless the GIF has a color profile.
JPEG	jpegtran from libjpeg-turbo	Optimized Huffman tables and progressive encoding, without touching the image data.
JPEG XL	cjxl / djxl from libjxl	Lossless JPEG XLs are re-encoded losslessly at a higher effort (typically a few percent smaller). Recompressed JPEGs are rebuilt and recompressed, and still turn back into the exact original JPEG. Lossy, animated and rotated JPEG XLs are left untouched.
PNG	oxipng	Lossless filter, bit depth, palette and deflate optimization. Animated PNGs are left untouched.
SVG	SVGO	SVGO's default preset, run in the built-in Jint engine (no Node.js needed). SVGs are text, so they aren't pixel-compared.
WebP	cwebp / webpmux from libwebp	Lossless WebPs are re-encoded losslessly. Lossy WebPs are never re-encoded, only stripped of metadata.

By default metadata (EXIF, comments, camera info) is removed, but color profiles and JPEG orientation are always kept so images look exactly the same.

Settings

• Formats to optimize (AVIF, GIF, JPEG, JPEG XL, PNG, SVG, WebP)
• Remove metadata (on by default)
• Maximum compression: Zopfli for PNG, maximum effort for WebP, AVIF and JPEG XL, multipass SVGO (off by default; much slower)
• Use progressive JPEG when it's smaller (on by default)
• Keep each file's original "Date modified" (on by default)
• Check for updates when the app starts (on by default)
• Add "Optimize with Image Optimizer" to File Explorer's right-click menu (on by default when installed)

Settings are stored in %AppData%\ImageOptimizer\settings.json.

Install

Download ImageOptimizer-win-Setup.exe from the latest release and run it. It installs for your user account only (no admin prompt), adds Start menu and desktop shortcuts, and installs the .NET 10 Desktop Runtime first if it's missing. It also adds the File Explorer right-click entry for your account. Uninstall it from Windows Settings > Apps, which removes the entry too.

When the app starts it quietly checks for a newer release and downloads it in the background. Once it's ready, a Restart to update button appears in the footer; if you'd rather carry on, the update installs the next time you open the app. You can turn this off in Settings.

Requires Windows 10 or 11 (x64).

If your antivirus blocks it

The installer and app aren't code-signed yet, and the app rewrites many image files in quick succession and runs bundled command-line tools, which some antivirus behavior checks mistake for ransomware. If yours blocks or closes it, add the install folder (%LocalAppData%\ImageOptimizer) to its exceptions and report the false positive to the vendor (for Trend Micro, submit the file for analysis; on a work PC, ask your IT team).

Releasing

Bump <Version> in Directory.Build.props, then open Actions > Publish > Run workflow. It uses that version (or the one you type in), tags the commit vX.Y.Z, builds and tests it, packs the installer and update packages with Velopack, and publishes them as a GitHub Release. Installed copies pick it up on their next launch. Pushing a vX.Y.Z tag yourself does the same.

After publishing, the VirusTotal workflow uploads the installer and the app's own exe and DLL to VirusTotal and lists any antivirus engines that flag them in the run summary, so false positives show up before people hit them. It needs a free VirusTotal API key saved as the VIRUSTOTAL_API_KEY repository secret (without one it skips the scan), and it never fails a release. To scan an earlier release, open Actions > VirusTotal > Run workflow.

Building

./scripts/fetch-tools.ps1          # downloads the pinned optimizer binaries into ./tools
dotnet build ImageOptimizer.sln
dotnet test --solution ImageOptimizer.sln
dotnet publish src/ImageOptimizer.App -c Release -r win-x64 --self-contained false -o publish

Every push is built and tested on Windows by GitHub Actions, and the ready-to-run app and its installer are attached to the workflow run as the ImageOptimizer-win-x64 and ImageOptimizer-win-Setup artifacts.

The engine (src/ImageOptimizer.Core) is cross-platform, so its tests also run on Linux or macOS with oxipng, jpegtran, cwebp/dwebp/webpmux, avifenc/avifdec, cjxl/djxl and gifsicle on the PATH.

The tests include FsCheck property tests (PropertyTests.cs) that fuzz the engine with random and damaged images, checking that nothing crashes and that a file is only ever replaced by a smaller, verified copy.

License

Image Optimizer is released under the MIT License. The bundled tools keep their own licenses; see THIRD-PARTY-NOTICES.md.