I always let PGV create thumbnails except I just uploaded an image and want a different size thumb than was generated. Not sure method to upload the thumb I created after-the-fact.
You have the option while uploading to select a different file for the thumb, and that's when I usually do it. I use a more cropped version for many profile photos. However, if doing it later, simply load the thumbnail into the thumbnail folder (same name as the main image).
How do I declare a thumbnail for my .pdf in the raw media format? I have the thumbnail, I have uploaded it . . . how do I link the thumbnail file to the .pdf in the raw gedcom code for the media item?
You can create a jpg/gif/png as a thumbnail of teh PDF and just name it with the exact name as the file including the .pdf and it will work in most browsers. I believe that some Mac browsers may not display it. This is in the readme.