This is actually supported by Game Maker Server(I've tried it myself). It's basically involves in;
*Creating a surface and drawing the image onto it.
*Passing the surface as a buffer : buffer_get_surface(buff, image_surface, 0, 0, 0);
*Encoding the buffer and converting it into a string var b_str = buffer_base64_encode(.....)
+Send it through Game Maker Serve gms_p2p_send...
+Receive the string and buffer_base64_decode(.....)
+Write the buffer onto surface with buffer_set_surface(......)
+Draw the surface draw_surface(...)
You can send screenshots or drawings to other users by this method. It's not very efficient, but it's the only way :)