I'm using a iStock image that's 849 x 565px (they're smallest photo size) for my Facebook Page, how do I change the image size for my Facebook Cover picture without distorting or affecting the quality of image?
You should be able to just add it and choose the position/resize itself. Because Facebook requires an image that is: 851x315. What you can do is (if you have Photoshop) is to create a new document with the size 851x315 and paste your iStock image in it and resize it to fit that size.
There are various online tools available online to resize image without losing its quality. Here is one of them http://www.picresize.com/ hope its help you