There's insufficient information here to provide an answer. Can you explain in more detail what the problem is? It sounds like you're downloading a bunch of images into a zip file and then trying to add those to products (which you can't do). Is this correct?
All product details in the store are pre loaded (via an app).
What I need to do next is have images attached to these products automatically.
The issue im having is the images are hosted on an FTP server in a img.zip file. The image file names match the product barcodes however I am unable to find an app that can actually automate the process.
This is do-able via an app you develop to run on a server elsewhere (outside Shopify). This app would have to be fed which images are to be updated - either by the presence of a zip file, or by being input a csv of products to update. It could then delete all current images for the product, fetch the zip file, expand its contents, push each image it found onto the product (https://shopify.dev/docs/admin-api/rest/reference/products/product-image) and then delete all the temp files and move onto the next one.
The app can be developed in whatever language or framework you like so long as it has the capability to ftp to download the zip files, expand zip files, and then make requests to the Shopify API.