It's totally up to you. With some boyfriends I managed to throw away a lot of stuff that with other boyfriends I would keep. I have a big box with everything that I've kept from all my boyfriends. Longer boyfriends have more things, as do those that mean more. When I split up with a boyfriend, the things I've kept go in the box, and I sort through the box to see what I have that no longer means anything. That's when I throw them away. I wouldn't delete the pics on your computer. Put them in a file of their own. I have sections by terms at uni (ie michaelmas, lent, etc) and there's a pre-uni file as well, which has a few pics of ex's. Sections at uni have pics of people I've dated and there's no way I'd delete them, because some of the memories are awesome. Best thing to do is to fill up the rest of your photo section with pics of you having fun with your friends.
Don't worry about bumping into her. Just smile and say hello, ask how she is. If she ignores you or is rude, that's her problem and she should get over herself, but I'm sure she won't be like that.