It looks like there's a new (optional?) dependency on the Image::Size Perl library that was introduced with the MT5 beta 4 release. I encountered this dependency when uploading an image asset to a blog entry. This library is not listed in the collection of optional modules in the System Information screen (System Overview -> Tools -> System Information).
gf

You should file a bug report @ http://www.movabletype.org/feedback.html
Reported here:
http://bugs.movabletype.org/default.asp?102925