Unable to create scaled Thumbnail image

After upgrading to drupal 5.6 and now 5.7 the images in my image gallery no longer display. I had similar error messages to that described in this thread: http://drupal.org/node/215455

... and I also had an additional "Original" in the image settings admin, added reasonable image dimensions, and clicked reset. In contrast to the outcome in the referenced link above, my installation displays no images, but instead displays this:

Image galleries

* Unable to create scaled Thumbnail image
* Unable to create scaled Thumbnail image
* Unable to create scaled Thumbnail image

* The derivative images for color envl have been regenerated.
* The derivative images for Needle Abrasion have been regenerated.
* The derivative images for 10xVessels have been regenerated.

Please help!

Bob Solimeno