At the finished size, say 640x480 or 640x426 or 640x512, do this:
Ctrl-A to select the whole image (or use the rectangular marquis tool to drag entire image), then go edit, stroke. You will be presented with a box to fill in the size you want, say 10 pixels. That will give you a 10 pixel wide border. There are three buttons below, Inside, Center and Outer. Check the inside button, as you want to add the border on the inside of the selection. You also have a color selection you can click on and select the color of your choice. Recommend simplicity of white or black for challenges. If you want to add a second color, say a 1 pixel wide line at the edge of the now stroke white border for instance, zoom way in and to to one corner. Use the rectangular marquis tool again and click at one corner at the division of the image and border, and drag the box to the opposite corner. Using the stroke command again, select 1 pixel as the size and select inside or outside, and again the color you want from the color palette. Once done, click anywhere to de-select it, and view image actual size to see what it will look like.
The canvas size method also works, but there is some resizing involved and can degrade the edges at times. |