Fellow commenters already bashed GIMP - kudos to them
...still can't resist myself.
GIMP is good for occasional image resizing or cropping, maybe even for applying some filters, but it royally sucks for non-destructive editing. Layer effects? Nah, who needs them. Ability to lock mask position while moving the layer content within it? That's for lazy Adobe fanbois. Who am I kidding, ability to preview layer during transformations, to do image editing, like, visualy, was added when GIMP celebrated it 20th anniversary. People who advocate GIMP must never have tried to use longer than 10 minutes.