Joey wrote:PGE's tool for taking screenshots of full levels would be a good way, but I've never used it so you'll have to consult the PGE documentation or the Help and Support forum if you need help with it. You can also take multiple SMBX screenshots and then edit them together with Paint.NET. With either method, you'll need to crop them (not resize by stretching) to 960x480px.
Here I have been explained how to easily take level screenshot of the fixed size: http://www.supermariobrosx.org/forums/v ... 91#p242691
But you required taking a laboratory build where this was being implemented (after fixing some set of reported bugs I'll release it officially to begin huge reorganizing job after)
You can copy-paste this to the main post (into the spoiler) as quick guide of most easy way