i choose my thumbnails from my videos too..well..sometimes lol here lately i've been getting a little more creative. Anyways, when you save, save as a PNG. PNG is like what jpg,gif, and jpeg is..cept it's not, it's PNG <-- lol
As far as I know, .png is known for saving files as the highest quality. you will happy to hear that there will be 0 Grain in your photos. Grain usually appears when you save as a jpeg or jpg. jpeg or jpg = a no no. .png = a yes yes
i wish you the best of luck and happy thumbnailing!
ps - i use pixlr.com/editor for image manipulation. its dabomb