This tutorial offers two basic strategies to add borders to your images. The two strategies represent two distinct logics of achieving the same result: (1) creating a mat above your image and cutting out an opening in it to see the underlying image, or (2) modifying the canvas that underlies your image. The choice of the option depends on the image you have.
If you already have an image that is perfectly cropped and framed and you like it as is - proceed to option 2 below (easiest way to add a border in 2 easy steps). If your image needs some cropping, go ahead with option 1. Of course you can just crop your image and go to option 2!
Option 1: If your subject matter is not too tight for cropping - overlay a "mat" above your image
Step-by-step summary
Decide on your preferred output size (if you are adding borders for printing) and set the dimensions of your crop toolOR crop down to your favorite composition (for display on the web). Don't crop too tight as you'd need some extra space for the borders.
Create a new layer
Fill the new layer with your preferred border color.
Keeping the new layer active, make a rectangular selection using the marquee tool and detele it.
Flatten the image. Voi la.
Sizing and cropping your images
Most of you probably just want to print a 4*6 or a 5*7 or an 8*10 of your favorite photo with some sort of border added to it. What you have to remember is that to make sure that your image prints exactly the way you want it to print, without getting cut off on a side or at the bottom, you must make sure that the photo you got from your camera has the right dimensions for printing. If that is not the case, you will have to crop it down to the right size.
Crop your image
Select a crop tool I have this picture of my cat Daisy (above) that I want a 5*7 print of, with a nice border of course. Step one, select the crop tool and set its dimensions to 7 inches for widths and 5 inches for height. Then, start at the corner and drag the crop tool across the picture. You will see that the area that you drag over "opens up" and becomes fully visible. Some parts of the picture will remain shaded - these are the areas that will be cut off after you hit "enter" to make your selection. Experiment with the crop tool until you are happy with your final selection.
my crop off I like a tightly cropped close up of my image to emphasize Daisy's face and her incredible whiskers:
After you are done cropping, it is time to start adding the border to your image. The steps that we do in photoshop follow the logic of "matting" a picture if you were to frame it for display on your wall: you have a print out of your image and you overlay a mat with a cut-out window above it! That is exactly the effect that photoshop achieves when you create a new layer and "cutout' a part of it to make the underlying image visible.
http://www.graphic-design.com/photoshop/tutorial/adding-frames-and-borders-photoshop