I recently had an issue where we had changed the thumbnail size in VM2, yet the thumbnails for existing products all remained the old size.
Instead of manually updating each product - a big pain - I just did the following:
- Go to the images/stories/virtuemart/products/resized folder. Delete all images in this folder.
- Also go to the images/stories/virtuemart/categories/resized folder and delete all images. Now when you reload your web site from the front-end the thumbnails will be recreated at the right size for both product pictures and category pictures.