There are a lot of photo editors on market, many of them open-source but I have yet to find one that does what IrfanView does so well — photo processing.
I call it processing because it doesn’t replace a full-feature editing program like GIMP (or Photoshop).
If, for example, you have 600 photos from your students that are all too big and a few too small — IrfanView can resize them with ease. The same for a bunch of photos that all are poorly exposed. Or a directory of photos that all need to be rotated. Or converted from GIF to JPEG.
IrfanView is also a very efficient file viewer where you can view a batch of photos and tweak them one-by-one. You could do this with Photoshop or GIMP but not as efficiently. It also works nicely for batch scanning.
IrfanView is not open-source but it is free for individuals. I include it here because I have not found an open-source program which replaces it. If you decline to have the adware installed, it is as well-behaved as most open-source packages.
If your library is running Windows PCs, I consider it a must-install.
IrfanView is only for Windows but it runs fairly well in Linux using Wine (a Windows emulator). Reportedly, it can run on a Mac using a Windows emulator.
Portable version: http://www.irfanview.com/
Description: a photo processor for resizing, cropping, rotating, sharpening, batch-scanning, creating panoramas and much more. Many features can be done in batch mode.
License: free but not open-source.
Alternatives: many, notably GIMP.
Rating: very useful for Windows users