Note: This review addresses v4.26 of the software.
It's this simple: If you ever use graphics for almost any reason, you need this program. It's small, it's fast, it's free, and it does its work better than any competitor. What more can I say?
Irfanview's main task is to view graphics files, and it does a superb job of it. You can view just about any file format you can imagine, and display it lightning fast. It can also display thumbnails, show your files as slideshows, and play multimedia files. Given that it's a file viewer, the program also includes a surprising number of graphics editing tools, including adding effects and cropping. And if you need to convert files from one format to another, it can't be beat. It's particularly effective at batch-converting large groups of files.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.