most likely, they start with a machine print (A photo) and then have someone embellish it by hand. the other possibility is that they're selling reproductions painted by students as a learning exercise - they're really too cheap to be anything else.
most "prints" you buy of paintings from an art gallery are embellished like that...some will even be marked "hand embellished by the original artist" (but since this site is dealing in big paintings by dead people...)
if it's embellished, you can usually tell if you look close, and have some past experience looking at real oil paintings. but it depends on how much time they put into the embellishment