I use an ancient version of Photoshop that I got on eBay. If you use Windows, you might consider a used Photoshop 7 because it doesn't require registration or "activation" by Adobe to run. If you have a Mac, Photoshop CS requires no activation, unlike with the Windows CS version. Most digital art on FA is done with Photoshop 7.

Can GIMP add an adjustable amount of uniform noise?
Does it have smudge?
Does it have something like Photoshop's Levels?

If so, you can do this:
http://www.furaffinity.net/view/378584/
See the minitutorial under the story.
http://www.furaffinity.net/view/403805/
See the minitutorial under the story.

If you have any questions, I'll do my best to answer them.

just one question....how long does it take?

The first time that you do it, it is likely to take several hours, but it becomes quicker with practice.

With...or without a tablet?

With a tablet. I use a WACOM Intuous 3, but their new Bamboo Fun has gotten a lot of user praise and actually has higher resolution (but doesn't measure pen tilt, just position and pressure). It comes with Photoshop Elements 6:
http://www.wacom.com/bambootablet/bamboofun.cfm

$99.00, which is less than half of what I paid for mine. Moore's Law is GOOD for digital artists! I tried painting with a mouse, and after 20 minutes, I decided that buying a tablet would be a lot cheaper than carpal tunnel surgery, which is where I was rapidly heading with all that mousing around. A tablet is a lot more comfortable, much easier to control, and vastly faster. I'm completely satisfied with my 4"x6". Much to my surprise, 99% of the stylus wear marks are in a 2"x3" area!

and I use a mouse, why....those tablets cost too much and since I dont have a job yeah!