Karlie here from the Shopify Guru Team! Excellent question! There are a few different apps that come to mind when adding in a customization functionality to your product page. If you're looking for a way to have your customers upload an image with their order, then you could use an app like;
Now, none of those above options would allow the image to show up on the product itself. If you're looking for an app that would allow your customers to upload their own images and see a live-view of the personalized product mock-up from your website, then you would want to look into an app such as;
Give some of these apps a look and let me know if this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!