Just recently I've ran across many blogs with what I'm sure the author thinks are image thumbnails. I have just timed one page at +60 seconds to load and I'm on a 512K line, yet each image was only about 200px square.
The file size of some of these images though is ridiculous - anything from 150K + (One such small image was 450K)
You might have tons of bandwidth, and you might have a T3 line and you might not see the effect because you use the cache, but believe me, visitors will just click away before bothering to read.
Use REAL thumbs, don't just link directly to the main image and force it into a smaller size.