I think the easiest method would be to use a 3rd party cloud hosting (similar to dropbox…etc) that would allow you to offer the upload option on any page, and just add the link in your page… it probably can be done through Google Cloud, AWS S3 as well or with some Wordpress plugins (the options can be endless).
Step 2:
Once the user has uploaded the picture, just send the picture URL (from your cloud hosting) into Shopify as extra data (in notes, or attributes).