The alternative text will explain the image content. Then images are heavy. When you write a text only web page, say about 5Kbyte of size, you are writing quite a long page. On the contrary an image of about 5kbyte of size isn't a big image. Just a small button for example.
Photos are very heavy HTML elements.
A photo maybe 10k, 100k, 200k or bigger. The GIF format was developed by Compuserve and it stands for Graphics Interchange Format. This format is available for several platforms and so it is much used on the web. GIF images can use up to 256 colors only, but their big trouble don't concern colors. In that case JPEG format is better than GIF format.