Ok, I found out it has definitively something to do with the "Design mode could not load swiz-framework-1.0.0-RC1.swc. It may be incompatible with this SDK, or invalid. (DesignAssetLoader.CompleteTimeout)" warning. This somehow affects the loading of the mx:Image.
Recompiling the SWC might work with the swiz swc, but there are more swc's giving this error like the google maps swc which does not have the source avalable freely.
This is a real pain as I like to use a mockup image in Design Mode to place components exactly as provided by the design team.
Any solutions for this? I'm having this same problem using the Mate SWC. As soon as I include the Mate_0_9.swc then none of my images load in design view and I get the warning "Design mode could not load Mate_0_9.swc. It may be incompatible with this SDK, or invalid. (DesignAssetLoader.CompleteTimeout)"
I've tried compiling the library from source but no joy - any workarounds?
I was unable to find a solution and removed all dependencies to the offending swc. Bit of a shame but I badly needed visual feedback.
I had exactly the same issue, but on recompiling my SWC with Component Set changed to MX Only (was MX + Spark) under Flex Library Build Path/Library Path, the warning went away and I can work in Design mode again in the project referencing the SWC.
No idea why it worked, but it worked...