Well I tried Monster.com's salary thingy:
http://littleurl.com/?047e
They didn't have Photo Retoucher so I chose Artist. The median salary is around $54k. Which is about $27.00/hour. But this is based on a full-time job with benefits.
If you are freelancing then there are expenses you need to keep in mind: Travel, computer, health care, office, electricity, etc. Even if you work out of your house, you need to calculate the cost of the space you use. I have a home office. It's one room out of a total of about 10 roooms in my house so I expense it as 1/10th my overall cost. For instance, if my mortgage is $1500/month then my office costs me $150/month.
These are just ideas. You'll have to figure out on your own how much you're worth. Just make sure you can cover your costs and still keep some of the money.