This extension is the best and easiest one I found that converts a web page into a PNG file. The original service that the plugin uses is called IWeb2Image and it converts a web page given its link into an image. Towards the bottom of the page there is a link to a chrome extension that communicates with the service directly.
All the other extensions that I used before had their own troubles, including high cpu usage. I hated almost all of them. Safari used to have an export menu in the File menu, that exported to image. It seems that they have removed it a while back as well.
This extension will not convert pages based on login sessions as it sends the page url to sciweavers IWeb2Image service to convert it, which will essentially attempt to connect directly to the URL.