if you love your gf tell her everything now...
start with the beginning and tell her every detail...
dont lie one bit about any of it and MAKE HER LISTEN TO EVERYTHING before she goes off on a rampage and all hell breaks loose...
if she only listens to "your mom is sending me naked pictures" and doesnt hear the whole story in the end you'll end up getting blamed for it too and your ass will be on the curb with neither of them..
don't just show her the pictures then say your mom sent me these...
put some actual thought into how you can tell your girl in a way that shows her you care about her and want to be with her.. cuz this will probably shake her foundation if she isnt in on this[the set up theory some were discussing]
BUT if you can manage keep things the way they are and hope everything stays the same
OR if you want to try your luck go for the mom... fuck her and her daughter and enjoy life as how many guys to to fuck a hot mother/daughter combo?
and G.I.M.P [google it] is an open source[free] version of photoshop basically..
so if paint is too hard for you to cover her face you can use that... plenty of tutz on youtube on how to use gimp and even make gimp look like photoshop