Another odd problem I had when migrating a Wordpress blog to a new server is that some images would not display. It wasn't that no image were showing; only some did not display. I have not found the root cause, but a solution.
It turns out that something happened to the thumbnails, i.e. resized versions of images. The easiest solution was to force Wordpress to regenetate them.
For this, I used the Regenerate Thumbnails plugin by Alex Mills. Make sure to disable the option "Skip regenerating existing correctly sized thumbnails (faster)."
After the plugin completed, all images were showing correctly again.