It seems to me there are several issues here, and I am going to go over them one at a time.
One, you refer to it as "eyecandy" therefore I am going to assume that it is of a non-pornographic nature. If you "found" it on the computer 3 years ago and she deleted it, you probably made a fuss about it being on there so it doesn't surprise me that if she had more that she would hide it so you don't fuss about it again. If she is receiving this eyecandy in an email then its not like she is actively searching for it, which leads me to wonder what you are doing in her email, but thats neither here nor there.
Perhaps she saved it to send it to some of her friends that she thought might appreciate a little "eyecandy." Did you have a talk with her about this subject? Maybe she didn't realize you have such a problem with what her friends send her. I can't answer why she personally feels the need to have eyecandy, thats a subject you will have to approach her with, but be prepared to endure any backlash you may encounter when you have to admit that you were going through her personal files when you found it.
The second topic I would like to cover is that you said she wouldn't like it if you had "eyecandy." Do you know this to be a fact? If she has it, I am guessing that she may not mind as much as you think.
Also, you said that she has low self esteem, and that you try to give her compliments to "help her." This sounds quite patronizing to me. Maybe you are the one with low self esteem if you are so bothered by some harmless pics to the point that you "find" (actively search for) them in her email. It sounds to me that it isn't the "eyecandy" that is the issue here, but rather a trust issue which could be the root of her low self esteem.
Eyecandy in most cases does not mean that the viewer is having adulteress thoughts about them. Besides, you refer to your partner as your girlfriend and not your wife so its not really adultry. If she were out with other guys, it would be cheating, and looking at pictures isn't cheating. I wouldn't worry so much about it if I were you.