I am not sure if this is a problem because I use the plugin
and it will get remote images i link to in my post and store them on my site and the image names can be rather long.
then the Post Thumb generates thumbs based on this
others are like
files i upload by ftp and embed with a url are like
Anyway about 10 to 20% of the first images in my posts are not being thumbnailed. They do not show up and the file thumb_thumbnail.jpg is being used...
however I have my default set to thumb_default100x100.png
I do not use WordPress to store my images either they are grabbed and cached or i ftp them into a folder.
All first in post images are local to the server prior to the thumbnail being generated.