Head is spinning over GIMP. HELP!!!

I just spent the last 45 minutes trying to uniform all of my photos, but haven't even done one because I have forgotten how I did it the first time. Will someone please tell me how to darken the edges like I have done previously in these photos.

Yup, you select either the rectangle or circle (your choice) and align the box with your border, click on the tiny dotted square on the bottom left (near the bottom scrool bar) to add "mask" (which will be red). Go to filters > blur > gaussian blur, set the blur radius to AT LEAST 100, click OK. Go to Select > invert. Go to Edit > Fill with FG color. The darkened edges are actually called a vignette and you can use any color you want. Sometimes white is very nice. Have fun! I love GIMP!