Just ran across a problem where two JPG images were intended for display in Flex 1.5 using the mx:Image tag, but only one image would render in the browser. Using a clue from a Flash Player problem, I opened both images up in PaintShop Pro and saved them again but as Standard JPG and as Progressive JPG. Only the Standard JPG images would render, but not the Progressive JPG. Standard and Progressive are options when saving JPG files in most image editors. So if you find this problem, try saving the image again but in Standard JPG format and Flex will be able to render it.

"1.0" encoding="utf-8"?>
"http://www.macromedia.com/2003/mxml">
"test1_standard.jpg" width="150" height="150"/>
"test1_progressive.jpg" width="150" height="150"/>