I used to use Irfanview to edit my files but it stopped working a couple of days ago so I switched to GIMP but I can't seem to find a way to save a file other than xcf.
You need to select file => save as => select type by file extension => jpg or any other extension you want
In fact in GIMP you save your file as xcf and you export it to the file type you want (including all major file types).
You can use SHIFT + CTRL + E for this purpose or simply "File / Export." Saving the file in xcf allows you to do a lot more editing if you reopen the file later.