Assuming you're running Windows, IrfanView was the canonical freeware solution for this sort of thing back when I was still using Windows as my primary desktop. It looks like it is still being maintained/enhanced. It started out life way back in the day as an image viewer, but it has quite a few image editing and conversion features as well.
There's also GIMP, which is an Open Source, cross-platform photo editing tool for Linux/Windows/OSX. Personally I find the user interface rather counterintuitive at times, but it is quite powerful.
Depending on how sophisticated your needs are, and your tolerance level for squirrely UIs, one of the above two should get the job done. In fact, why not just download them both and take them for a test drive?
Edit: And Firestarter's recommendation (Paint.Net) is popular as well.
The years just pass like trains. I wave, but they don't slow down.
-- Steven Wilson