Image file formats on computers can be categorized into two different kinds, the first kind being called lossy, the second kind being called lossless.

Saving an image in a lossless format means that none of the original information as intended by the creator of the image is lost.

Only lossless formats are suitable for storing pixel art whereas lossy formats will destroy some of that information in favor of a smaller file size and or faster image decoding speed.

further reading:

• Image File Compression