I'm far from an expert on the subject but...
I know that jpegs are a compressed format. I also think that the amount of compression (more compressed = worse image) is adjustable. I think... Working with imagery in Matlab had left me with this impression.
Now, I'd assume your Digital camera would provide the best image it possible could, assuming it renders to jpeg. Could geocities somehow be altering the compression setting of jpeg images during upload or on the fly? I don't know.
BTW, the mars reflection link on your page is linked to the Volcano image.
et
