A truer image.
BMP files are sometimes known as “true images” because they render each individual pixel within the file. They don’t compress automatically. This makes them large — a collection of BMP files will rapidly consume lots of memory and space.
PNG files display high-quality images, but unlike BMPs, they use compression. They work well for simple graphics like icons, logos, line drawings, comic strips, and embedded graphics.
File size.
Given the same image or content, BMP files will be larger than PNG files. Both retain as much detail and information as possible, but PNG files automatically compress — and can be compressed again to make the file even smaller. Because of the way they handle individual pixels, BMP files have dimensional restraints, maxing out at 4GB.
Common uses.
BMPs store high-quality images with a maximum amount of detail. When they first launched, this made them ideal for editing. Later file types have surpassed BMP for editing use cases, so today most people use BMP files for storage.
The original purpose of PNG files was to move images around the web. A staple of many websites, they’re a good file type for small illustrations, sketches, drawings, logos, and icons. Graphic designers often use PNGs for stand-alone graphics.
It’s particularly easy to use transparency in PNGs when there’s a singular subject with defined borders and a large block of surrounding space, like a logo or icon. This allows graphic designers to use them, both with layers and as layers, when rendering backgrounds.
Compression.
Both BMP and PNG files are lossless file formats. This means after compression, you can restore either file type to its full original quality.
BMPs use a simple algorithm to efficiently compress images into small sizes. PNG files compress quickly to small file sizes — plus, you can compress them multiple times.
Quality.
BMP files have slightly higher quality due to how they handle pixels and their raw format. However, there's not much difference in the quality between BMP and PNG files.
Printing.
Neither BMP nor PNG files are good choices for printing because both have limitations with physical paper and ink. They especially struggle with the CMYK (cyan, magenta, yellow, key/black) color scale.