I am using GD2 for thumbnail generation.
I figured it was using the permissions from the base uploads within IPB.
So I suppose one of two things need to happen. In IPB if someone uploads a .bmp image, the forum simply gives a link to view the file (does not generate a thumbnail) however, in the registry it shows a broken image and a link that simply takes you to the registry home page. Wonder if there is anyway to fix that?