Problem Uploading Images Using the Applet

Gallery URL: http://www.roverte.com:8008/gallery/
Gallery version: 1.5.2-pl2
Apache version: 2.0.54
PHP version: 4.4.0
Graphics Toolkit: Image Magik
Operating system: SUSE 10

I've installed gallery, and when I copy images to my server using scp, I can then add them to the gallery using the URL upload method without a problem.

However, when I try to upload the same batch of images using either of the applets, I get an error for a lot of the images:

"Upload error: Uplaod Failed: 'Invalid image: image_name.jpg'."

Any ideas on what might be causing the problem?

Thanks,
Alex