Post Image Not Showing Image

My site can be found at http://www.mildlyhotpeppers.com.

Short version: Thumbnails from post image aren’t showing up, and instead the default image is appearing even though the post itself is only one image. (It does work for four posts, though.) Can anyone explain in detail how the plug in works, or what the plug in is looking for to display the image?

Long version:

I’ve scoured the forums, but I can’t seem to find an explanation for this:

My site is a web comic, and so I’ve used WordPress as a base for dynamically generating pages. Essentially, each post is really just an image tag. I plan to create an Ajax navigation bar, where visitors could scroll sideways to browse through the comics. I didn’t want to use the actual comic with <?php the_content(); ?> since it would take up too much space. Instead, I was hoping to use the thumbnails associated with each image.

Thus, I found post image, and it seemed to be a great solution. I found, however, that the image only showed up for four of the posts. The others defaulted to the image I specified, apparently unable to find an image inside the post. Obviously, though, that isn’t right because the entire post is just one image tag.

Can anyone explain (the creator, maybe?) how the plug in works? I’ve tried to look at the code, but I just ended up with a headache. Maybe then I can figure out why it’s not finding a thumbnail to show.