Gallery thumbnails (some) are pixelated and grey

Problem pages:
http://www.cartoonistorian.com/2011/07/greenville/
http://www.cartoonistorian.com/2011/02/seattle/

Good pages:
http://www.cartoonistorian.com/2011/01/olympia/
http://www.cartoonistorian.com/2011/03/toronto/

I’ve successfully uploaded images to a WP gallery on this site (see above) but lately some of the thumbnails (see problem pages above) are coming out grey and pixelated? I looked at all the images that WP creates when uploading the original image and it looks like the thumbnail and medium sizes are affected but not always.

Can you help me troubleshoot this? I don’t know where to begin?