Adding a Border
Adding a border (or stroke) provides a finishing touch but judges generally don't like borders, especially large borders. If you feel your image is bleeding into the background, a two pixel border should suffice.
As with everything in Photoshop, there are a couple of ways of doing this. Once all your adjustments are complete a border can be added.
Using the STROKE command
Select all (CTRL A)
Under EDIT select STROKE.
The dialogue box will open and you can choose the width and colour of your border.
The check boxes INSIDE, CENTER and OUTSIDE allow you to position the stroke line inside your selection, centered on your selection or outside it.
For a simple border keep the blending mode to NORMAL, but do experiment!
This means that by selecting all your image, only INSIDE will give you the full width line you have asked for in WIDTH, since it “eats” into your picture by the number of pixels you’ve selected. CENTER gives you half the number and OUTSIDE doesn’t work at all!
You can, of course, use this to apply a line within your image, by making either a rectangular or oval selection and applying the stroke to that. Then the INSIDE, CENTER and OUTSIDE check boxes function according to the width set.
Adjusting the canvas size
This method adds a margin to your image by increasing the size of the canvas. By doing this repeatedly you can have any combination of thickness and colour you like.
Under IMAGE select CANVAS SIZE.
The dialogue box will open and, for simplicity, check the RELATIVE box. This means you won’t have to calculate the new canvas size, just enter the size you want. I find it easiest to work in mm.
For a simple border leave ANCHOR at its default.
Select the canvas extension colour.
Repeat to add more lines.
The ANCHOR selector allows you to add extra canvas, say, to the bottom. You might want to add a title for example and need space below the image. Note this method does increase your file size.
Finally, the STROKE command also appears under the effects (fx) menu in the layers palette and allows for other effects to be combined with it, eg drop shadow. To make fx available you need to duplicate your background layer.