Some folks have reported that regardless of what they do, their image uploader in 2.5 works in some browsers but borks in others. We debugged it over here at IdeaStar, and here's what we did to fix it:
1. Checked server error logs.
2. Determined problem was in the \wp-admin\includes\image.php file, line 138.
3. Went into debug mode.
4. Added the following line immediately BEFORE
$image=, knocking that line down one:
Uploader worked perfectly, cross-browser.
5. Commented inserted line out: it worked cross-browser still.
6. Deleted line, but left line 138 blank: it still works.
7. Deleted line, moving the
$image= back to line 138: it still works.
Conclusion? There's a line-break or a line-ending in there that's incorrect: perhaps a UNIX, or a Windows or a Mac thing, but it's not correct.