Julie here from Shopify Support.
Offering your customers the ability to upload, resize, and preview their own designs on your products is an excellent idea and definitely makes for a great customer experience. There are actually a few apps designed to handle this. Some popular options include:
These apps have excellent reviews and are perfect for merchants who sell customizable products. From each app's listing, you can read more about the app and its features, view screenshots, and even access example stores so you can see what the apps look like in action. You'll find an View example store button between the app description and screenshots. This is a great way to see how the app operates from a customer perspective.
Taking a look at each example store here on my end, I can see that there's an upload button on each product page. Once an image is uploaded, customers also have the option to resize and crop it. These apps are also capable of generating a preview of what the uploaded image will look like on the product.
It sounds like these apps offer exactly what you're looking for and I'd love to hear what you think!
I just wanted to follow up and see if you had a chance to check out any of those app suggestions.
Let me know what you think!