It's possible but you should have the problem with any browser if that's the case.
Start by adding a crossdomain.xml file to the sitewithpics server.

Code:
<?xml version="1.0"?><!DOCTYPE cross-domain-policySYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">
<cross-domain-policy>
<allow-access-from domain="www.mysite.com" />
<allow-access-from domain="mysite.com" />
</cross-domain-policy>
Also install the debug version of the flash player for both IE and FF and download flashlogviewer from Koolexchange. Then when you test you can see any errors and this will help you find any problems. You will wonder how you ever lived without them.

Email me the address of your test and I'll also give it a look, I also have tools in FF that lets me see the traffic between FF and the server. (Firebug plugin)