You can show pretty much any kind of graphic image, if the viewer has the tools to see it. There are 3 types of images, that are the most commonly used - BMP (Bit Maps), GIF (Graphic Interchange Format), and JPEG (Joint Photographic Experts Group). BMP files have the most detail but they take the most space and thus take the longest to download. Generally, don't use BMP files. GIF and JPEG files are roughly equivalent, but JPEGs are usually better at images with a lot of detail, such as photos. GIFs tend to be better at drawn images. Both are compressed versions of an original image, and both are "lossy" compressions - they trade losing some image quality for small size.

Here are 3 images of a graphics image. You can copy them by right-clicking on the image and selecting "save as". Then open the images up in something like PhotoShop and expand them to see the detail view of them. The first thing you notice is the dramatic change in size, from 38k for the bmp image to 7k and 5k for the compressed images. What you will notice in PhotoShop is that the .jpg file has a noticable loss of detail, while the .gif still looks pretty good.

BMP image JPEG image GIF image
bitmapped star image - note speckles in the red band on the shape on the left. jpeg star image - speckles didn't make it. gif star image - speckles present.

On the other hand, here are 4 images of a highly-detailed section of a motherboard. Again, there is a dramatic reduction in size, from 30k for the bmp image to 2k for the compressed images. What you will notice in PhotoShop is that the .jpg files will both have some degree of loss of detail but still look pretty good, while the .gif looks awful.

BMP image JPEG image
(middling compression)		 JPEG image
(max compression)		 GIF image
high detail bmp of motherboard mid-compression jpeg - pretty good detail high compression jpeg - fair detail really crummy image