.jpg are the worst to use because they change colors. It's better to use .png.
Look at the attached image. The picture is a screen capture of Paint Shop Pro. The image on the left side is saved as a .png the one on the right side is the same image saved as a .jpg.
I used a tool called the magic wand to highlight an area with the same colors. I clicked on the .png in the pink area then I clicked in the same area on the .jpg image. On the .png it was able to find the entire pink area but on the .jpg it could not because the colors were changed in the .jpg saving process. Using .png will also allow you to create transparent areas... something .jpg and .bmp don't do at all.