I have encountered this problem before, but it seems to be happening more often now.
I had two photos in my blog appear quite nicely for a week or so, and then for now reason, they become lost to the blog. Please see http://fencer.wordpress.com/2007/11/13/art-the-lazy-mans-way/ .
I find WordPress is not very photo friendly... I have a lot of problems sometimes getting photos and text to do simple things that I want them to do, like stay where I put them. But that may be a separate complaint.
Thanks for any light that can be shed...