You need to make sure your main image and your thumbnail are named differently.  I hold my Renderosity posting images in a special folder and name the main image "main.jpg" with the thumb named "mainth.jpg".  This way the two images are next to each other and easy to locate when uploading.  It sounds like your thumbnail image was uploaded as the main.

I hope that helps.

~Viv

Fractals will always amaze me!