There are two ways of image compression: lossy and lossless. If the information data can be compressed in a lossless way (like TIFF: zip-compression), none of the actual image data will be lost.

If a picture is compressed in a lossy format like JPG, some of the image data will be discarded for good. That does not mean that you can see the losses within the picture (100% JPG quality is very hard to distinguish from lossless compressed images). Lossless, low quality images will contain artifacts when saved.

none.

none.