On Qt5.0.2 I'm seeing imageData() return null when called on the mime data fetched from the clipboard, even though it reports that it has an image. Very similar to this report that was made back on Qt4.5:

http://www.qtforum.org/article/28190/mimedata-hasimage-returned-true-and-null-image-was-returned-mac-os-x-qt-4-5-2.html#post96689

The same code was working correctly on Qt4.7 & 4.8. As an experiment I tried calling clipboard.image() directly, but while it returned something other than null, pasting the resulting data into a webview did not produce any discernable results, so I assume it was an invalid image.