You actually don't need to do that. Below each image in facebook is an options button. 1. Just click on options 2. then click download and it will give you the full resolution that the person who uploaded the image used
Very useful tip, not just for this site but also for pics my lovely family in America who post great shots on FB but when I ask them for original resolution files to print for the old folks back here in England, they never get around to it.
Just noticed that if you open an image on Facebook in a new tab, and inspect the URL, there's often a resolution limiter. Here's one example.
http:// fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-xfa1/t31.0-8/p960x960/1596938_1510030165907606_9093664247853119232_o.jpg
I don't know why this is even there, but if you remove it, the image can scale up by quite a lot. The file size does increase of course.
Maybe ten people will read this and start using it, but I sure like it when the image fills the screen, should I zoom in!