It's not about pixels, but rather file size and format. I suspect your file size is too big, but I could be wrong.
Email me to badger@milsurps.com one of the pics that you can't upload and I'll check it for you, plus upload it to the current album as a test on your behalf.
The Microsoft Re-sizer program is excellent and when I "right click" 800x600 for resizing using it, the pics are a great size for viewing on the Internet.
Regards,
Badger