What is advantage of using bitmap images?

Raster images (such as bitmaps) are stored as individual pixels, each with their own color.

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To add to the earlier point made by a individual of, shall we say, few Words, I have composed my own answer.

Bitmap images are a collection of bits that form an image. The image consists of a matrix of individual dots (or pixels) that all have their own color (described using bits, the smallest possible units of information for a computer).

Bitmaps images are mainly used in photos due to their small file sizes, due to the fact that it only uses 0 and 1 to store their information. However, when magnified the pixels remain the same proportion and do not create more bits to compensate, resulting in you just getting a load of coloured cube with no particular correlation.

Basically if you want a picture that you're not going to magnify or change about much, a bitmap is right for you.