Once the card is added to your collection, you would go to the card's page itself. Then in the drop down menu where you can add multiple to your collection or for sale/trade, there is an option for View/Add Image at the bottom of the list. You would select this option, then upload your files for the image as normal. To my knowledge, it only works for autograph cards and the image would only be available for you, not the general public. Also, if you go the checklist or collection summary page, it will still show the original image, but going to the specific card's page would show your image. Hope this helps and if you have any additional questions about the process, feel free to reach out.
Matt Kenseth card collection is nearing 50% with over 2300 different cards. Always looking for the missing remainders to get to 100% completion.