See the demo and docs: http://nwdemo1.esri.com/ThumbnailBuilder.
This tool allows user to create thumbnails for ArcGIS Online items by merging a background and foreground image and some text.
TODO
You will need to install the proxy page. It can be downloaded from here.
To get the proxy to allow POST requests on IIS, you may need to enable HTTP Activation on your IIS Web Server.