Sometimes the card proxies are not enough. If you want to replace a card image/proxy with a new image, this is how you do it.
1. Become a subscriber
Visit http://www.octgn.net and click 'Subscribe'. Follow through and when you're finished, wait a couple minutes for the changes to propagate down to OCTGN.
2. Open up OCTGN
Make sure you don't open OCTGN as an administrator. Sometimes after an update it will launch OCTGN as administrator. Your best bet is to exit OCTGN and open it manually.
3. Open the deck editor
4. Opt to Create a Deck or Load a Deck
5. Select a card from your deck or from the card list on the right
6. Pick an image and drag it in
Pick a new image file from your computer and drag the file over the image of the card. If it accepts the image, you'll see a 'Replace Image' option show up, and you can just let go of the mouse button and it will replace it.
If your cursor changes to a circle with a line through it when you try a drag and drop
1. You may be running OCTGN as an administrator. Exit OCTGN and try again.
2. Sometimes you just need to try and drag, and it will fail, and you need to restart the program and try again for it to work.