in a nut shell:
if your a painter - scan them in nice, crop them clean, make sure they look as good as they do in person at 100% (close up). if they are photos make sure it's as noise free as possible. and make it interesting or it will be tough to sell. you do the advertising, add keywords descriptions etc.
is it difficult? depends on your definition. is it hard to take original shots and sell them - yes. i won't lie, it's not easy. many think selling art is a cake walk, but in reality it's a lot of work building a portfolio of work, and telling people about it, and keep filling it. beyond that getting the money is easy enough. i would say overall its time consuming.
---Mike Savad