Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Having used both GIMP and Adobe Photoshop I must say I personally believe GIMP is much better for common folks like you and me who need a relatively simpler set of tools to make quick edits to our Vacation pictures. Photoshop is undoubtedly more useful graphic designers, however for making quick tweaks, nothing beats GIMP. Recently GIMP 2.6 was released, so I decided to make this tutorial for those who still haven't discovered the magic of GIMP and don't know how to use GIMP productively.
1. Rounded Corners: There is something about Rounded corners which make Photos look very cool. So here's how you can give your photos rounded corners:
Open up the image you want to apply rounded corners effect.
Now select the 'Rectangle Select Tool'. It is in the GIMP Tools window.
Now select entire image or the part of the image you want to round off.
Now go to menu Select > Rounded Rectangle.... A box would pop up asking you to specify the radius in percentage. 50 is the default value. I'd recommend something like 15-20.