I use Photoshop CS6 at home, but on my work laptop (which I use throughout the day to post here while I should be working) I use
GIMP. It's a free image editing program that is just as good as Photoshop if you're just looking to make minor changes. It even does some more advanced things too.
I'm personally fine with having the limit on size be 800px at the longest edge, as long as it is truly enforced.
Those of us that follow the rules and take extra steps to size our image to the correct dimensions shouldn't have a disadvantage in size/detail compared to someone that submits larger size images. If larger sizes are OK, that's fine by all means, but then put that larger size in the requirements so that everyone can size their images accordingly. Regardless if whether you want to stick with 800px at the longest edge, or 1000px, etc., I think that the requirement should be enforced. If the requirement is 800px at the longest edge, and someone submits something 1200x500 px, it should not be allowed IMO.