This is regarding social networking profile pictures and instant messaging avatars:
I advise against using pictures of you and your significant other as your profile picture. The ideal picture if one of you, just you, and no one else. For obvious reasons, you want people looking at it to be sure which one is you. The less obvious reason is that by using a picture with other people in it, you're letting those people define you - and that's not good when that other person defining you is your significant other. A solo picture says "I may have friends, I may be dating someone, but I'm still my own person."
If you must use a picture with you and your boyfriend or girlfriend, follow these guidelines:
1) No kissing pictures. It's like a permanent PDA, and people can only see half your face.
2) Don't pick one where your S.O. is overpowering you. A lot of girls seem to choose a picture where he takes up most of the picture, and she's this little thing in the background you can barely see. Don't do that.
3) Center yourself. Crop the picture so you're in the center of the shot, and make sure the same rings true for your thumbnail picture. The thumbnail picture is small enough, you want to take up as much of it as you can.
Remember, it's your profile, and the picture should be of you! You can put all the couple pictures and makeout shots in albums or even post them directly to your wall. And certainly never make your profile picture as a couple shot just to impress the one you're dating, and they shouldn't expect you to do so. It's a nice gesture, but it's unnecessary in a healthy relationship.
And remember that you should never make your relationship your whole life. Let your significant other into your life, but a healthy relationship is just one small part of a well-rounded, fulfilled life.