What are lossless image formats?
Some compression-algorithms change the image to gain a better compression-ration - that are lossy algorithms/image formats. Most notably here is JPEG. Some keep all image information, these are called lossless. Lossless compression produces bigger files, but you have no changes to your graphic. Common lossless image formats are PNG, GIF or TIFF. As most of the time you can learn more from Wikipedia.