The one who request me this told me that the communication will be done by VPN.

"You don't mention what technology you are using to write the webservice. If it is something like .net, the you would be better of converting the native code into managed c++ that can be consumed by the .net service."

I know only C++/MFC ... this is not enough to complete the task ?

"If the MFC app rendered the images originally, then that rendering would be done on the client and supplied by a web server (which calls the web service to get the image data)."

Sound interesting ... can you elaborate a little bit ?