Hi Bob,
Document to PNG conversion is handled through a number of different methods depending on what your server supports. Some of these methods are more reliable than others and when no image can be generated the thumbnail falls back to the generic PDF icon.
Can you tell me which thumbnail generation methods are enabled in your settings Settings -> Document Gallery?
The other thing to note is that the each icon is generated in real time the first time that a gallery is viewed (then cached to be reused every time following). If you have a gallery with a large number of documents, DG avoids potential page timeouts by killing generation and using fallback icons after a set amount of time. The next time that page is loaded, generation is re-started where it left off and after enough views everything will have a cached thumbnail.
Long-term, the way thumbnails are generated may be configurable to allow generation upon upload, but I didn’t want to spend time on that functionality prior to this release and am not even sure if it is a good idea yet.
-Dan
