Joe, the reason I mentioned photoshop was because there is an abundance of learning support, some free & some very expensive as I am sure you already know, like you I was happy to pay for some of the help I needed but all the stuff I paid for was recommended to me by people I know & trust.
If you are going to do something like this then I am sure some people will be happy to pay a few quid for a good tutorial but how many you sell will be down to how well you promote no matter how good it is.

Andy, your first comment was correct 2 old farts lol