I'm in the process of doing this myself with my TTM collection. Here's what I do.
1. Go to the card that you want to add your own image of
2. Using the dropdown menu, the bottom option is "View/add image"
3. That will take you to an image upload page where you can either a) link to a URL or b) upload your own image.
I'm not sure if there is an easier way or a quicker way to do so, but this is how I am doing it, one card at a time lol.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.