I get an error when trying to load images

I just upgraded my OS X, and now every time I try to upload an image I get this error message:
File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.

I’m not doing anything different except I went from OS X 10.5.8 to 10.6.3. I size the images in Pixlr at 600 pixels wide, same as when it worked, tho’ I’ve tried different images at different sizes just to see if I can upload anything, and I can’t.