Replace speaker images with placeholder images

Updated speaker images to use a placeholder for the ISC 2025 event.

Author: rrrutledge

content/en/events/isc-2025.md

@@ -130,7 +130,7 @@ Chamindra de Silva is long time Open Source advocate and contributor originally
 </div>
 
 <div class="clearfix speaker-bio">
-<img alt="Clare Dillon photo" src="/images/events/speakers/Summit 2025/Clare Dillon.jpeg" width="200" class="speaker-photo" id="Clare_Dillon" />
+<img alt="Clare Dillon photo" src="/images/events/speakers/Summit 2025/White_Square.png" width="200" class="speaker-photo" id="Clare_Dillon" />
 
 
 **Clare Dillon** (State of InnerSource 2025)
@@ -140,7 +140,7 @@ With over 25 years' experience in the technology industry, Clare Dillon is curre
 </div>
 
 <div class="clearfix speaker-bio">
-<img alt="Daniel Izquierdo Cortázar photo" src="/images/events/speakers/Summit 2025/Daniel Izquierdo Cortázar.jpeg" width="200" class="speaker-photo" id="Daniel_Izquierdo_Cortázar" />
+<img alt="Daniel Izquierdo Cortázar photo" src="/images/events/speakers/Summit 2025/White_Square.png" width="200" class="speaker-photo" id="Daniel_Izquierdo_Cortázar" />
 
 
 **Daniel Izquierdo Cortázar** (InnerSource Value Metrics)
@@ -150,7 +150,7 @@ Daniel Izquierdo Cortázar is a researcher and one of the founders of Bitergia,
 </div>
 
 <div class="clearfix speaker-bio">
-<img alt="Fei Wan photo" src="/images/events/speakers/Summit 2025/Fei Wan.png" width="200" class="speaker-photo" id="Fei_Wan" />
+<img alt="Fei Wan photo" src="/images/events/speakers/Summit 2025/White_Square.png" width="200" class="speaker-photo" id="Fei_Wan" />
 
 
 **Fei Wan** (Reimagining InnerSource for the Agentic AI Era)