Example: The following image is taken from the Sammael Monster Gallery --
This is the URL of the image:
In the highlighted part, change the '1600' numerical value with another number, preferably above 3000. You should obtain a URL similar to this:
The last URL will lead to the full-sized image:
Apply this to the images that are 1200px wide or higher in the URL preview, and you will be able to save them in their original size. Enjoy!