What is GIMP?
GIMP (www.gimp.org) is a free raster image editor desktop application available for Windows and Linux. With it, you can open images in several formats, apply all sorts of filters to them, change contrast, brighteness, crop them, change their size, etc., and export them in several other formats, including JPG, transparent PNG, 8-bit PNG, GIF, lossless WEBP and lossy WEBP.
Due to how clunky GIMP's UI is, I personally wouldn't recommend using GIMP unless you need a feature you can't find in other libre software like Krita and Inkscape.