Within my web space I have a page of photos from a trip to Greenwich, all these files are JPEG format and have individual tags.
http://www.student.city.ac.uk/~abhj012/GI_Greenwich.html
Image formats differ greatly in the resulting image size, visual quality and compression. An image needs to be large enough to give enough detail, but not so large that it causes a site to open slowly. The quality of the image is very important for GIS but it is possible to loose some detail for the sake of space. For example, a road map can have one colour for the background as the need for elevation information isn't there, but a walking map needs elevation contours and colour differences to show if the area is flat or a steep gorge. Sites like Google with it's map and satellite images need to use the minimum size of files so that they don't slow down the users use of the site, but give enough detail. For the maps there are only a small number of colours used, while for the satellite images there are many more. As you zoom out on a image the number of colours also reduces.
To the right is a picture of a valley, taken from www.asiafrace.com/photos uploaded from Google Images.
The picture is in Jpeg format with a size of 84 KB
The image is clear and all colours are well defined.
The image used JPG compression, where you have the choice of the size and quality of the image stored.
This is the same picture saved in PNG format and is 135KB
The PNG format uses ZIP compression which is lossless.
From the saved JPEG image to the PNG one you can see a
slight difference in the sampling with a smoother image, but one with less contrast.
This picture is in GIF format with a size of 38KB
This file is compressed using lossless LZW.
The result is a smaller file but a grainy image.
The final image is 256colour bitmap.
The size of the file is 135KB, but the quality isn't good due to the reduced number of colours. The format is uncompressed and stored pixel by pixel.
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