There is a site where I changed a template which uses larger thumbnails. I changed the thumbnail dimensions in VirtueMart and enabled dynamic thumbnail resizing. I got my new thumbnails but VM keeps always recreating them. I reseted thumbnails in Tools&Migration which cleared the resized directory and file_url_thumb field in #__virtuemat_medias table.
Without dymanic thumbnail resizing I don't have thumbnails. With dynamic thunbmail resizing VirtueMart always creates the very thumbnail again and again (same filename, same content created in almost every minute).
Shouldn't VM store if the thumbnail is already generated and use the already generated file if it exists?
The thumbnails should only be generated 1 time. I checked on a website using Vm 3.2.12 and image have more then 4 days. and not regenerated each time.
So you have a problem with the image library on checking the file type or your template force to regenerate the files.