I think PhotoShop is essential for portrait artists. My 30 years as a graphic artist, the last 16 on the computer, have helped to learn photo manipulation. I compose photos together, change backgrounds, resize and move elements around, manage cropping, color, value and sometimes detail issues before picking up a brush! Furthermore, I like to show the client the results of the computer layout for approval before painting. Saves revisions at the end of the project! Most of what I've learned about PhotoShop has been by just doing it, but there are one-day workshops and books by Scott Kelby (
http://www.photoshopuser.com/) and I'm sure some local classes at colleges and private institutions, as well as online tutorials. I'm happy to answer questions if anyone on the forum needs help.