join two image

You can create a web service (REST service) that takes two images from the client, combine them on the server side, and then return the combined result image to the client.
You can send the image's as URI or base64String to the service, and return the result as base64String to the client.
On the service side, you can use a programming toolkit (leadtools) to combine the two images. You can find more details here:
http://support.leadtools.com/SupportPortal/CS/forums/44606/ShowPost.aspx
http://support.leadtools.com/SupportPortal/CS/forums/44826/ShowPost.aspx