Welcome to the Shopify Community! One option for allowing your customers to upload an image is to follow the advanced tutorial for file upload found in our Community topic Product pages - Get customization information for products. As noted in the post, this is an advanced tutorial, not supported by Shopify. However, if you have any questions regarding the tutorial, I'd recommend reaching out again in the Community to see if you can find some help from other users.
If you're not comfortable implementing this tutorial, you can always hire a Shopify Expert to custom code this into your store. Alternatively, you could use an app from the Shopify App Store, such as Upload-Lift Image Upload to add an image upload option to your product pages. You can search "image upload" to find other options in the app store as well.
Let us know how this goes!
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