A common way to do this is to host all your images in an S3 bucket and then just access the images by URL. Retool Mobile's Image component will support any image URL. Your implementation may depend on whether you know the URLs beforehand or need to query the S3 API to find them. It will also be more complex if you want to control who can access the images (anyone vs specific people).
This may help: asp.net - How to get the public URL of the image from AWS s3? - Stack Overflow
To test if the URLs work, just paste the URL into your browser on your phone. If it successfully loads an image, then you're good to go.
The thing you're Googling for here is something along the lines of "How do I serve images in an S3 bucket to the public internet".