Images should be optimized to load quickly. This means choosing the most appropriate file format and settings to compress the file.
Images often account for most of size of a web page. As a result optimizing images can often yield some of the largest performance improvements.
As a general rule, images comprised from flat colour (e.g. logos) should be saved as PNG or SVG files. Photographs should be JPEG images set to around 85% compression, which offers substantial size savings at a minimal loss of detail.