.bmp   -   Bitmap
The bitmap format, as used by Microsoft's Windows Paintbrush is the highest resolution available - the color of each individual dot (pixel) is recorded - but consumes a large amount of space which grows rapidly as the image becomes larger as the number of pixels is in proportion to the area, i.e.; the width multiplied by the height (e.g.; width=100, height=100, in 256 colors - number of pixels stored=10,000 (10K); width=200, height=200, in 256 colors - number of pixels stored=40,000 (40K); width=300, height=300, in 256 colors - number of pixels stored=90,000 (90K) ).

.gif   -   Compuserve's Graphical Information Format or 'GIF'
Developed by Compuserve to allow a medium level of compression for the transmittal over images over the internet, the GIF format is restricted to a color depth of from 2 (black & white) to 256 colors.

GIFs can be either interlaced or non-interlaced. There is little difference in terms of compression between these two sub-formats but a great deal in the way that they are displayed as they are downloaded; the non-interlaced immediately produces the end-quality picture during download but does so in strips from the top of the image to the bottom as the image is loaded. The interlaced sub-format produces an initial, full-sized, image on the screen rapidly by repeating a horizontal line of pixels and quality is increased as detail is filled in on subsequent passes.

Below are two examples of the two sub-formats taken from this site, the first interlaced, the second non-interlaced. Click on either to watch it reload (you will need to be on-line to do this);-

TIPS:   Web > Design The Javascript which follows replaces one image with another;- <img; name="name" src="filename1" onClick="Javascript:document.name.src='filename2'">