Step 1: Canvas size
Open your image and go to Image>Canvas Size. In the window that opens make sure the Relative box is ticked and change the units for the Height and Width to pixels.
Under Canvas Extension Colour select the shade you want to use. White, Black and Grey are already on the list but if you want to use red, for example, click on other and a window with a colour palette will pop open.
If you want to use a shade from your image you have to use the Eyedropper tool and click on the grass, bark or whatever you want to take the colour from so it is selected as your foreground colour before you change the canvas size. Then when you open the Canvas size window select foreground colour from the Canvas Extension Colour and hit OK.
Step 2: If you don't like it
If you think the line is too thick or thin, undo the changes in your history tab, by going to Edit>Undo or by hitting Ctrl, Alt, Z if you're working on a PC. Then simply repeat the previous steps, entering a smaller or larger number.
Step 3: Add another colour
If you want to add another line of colour to the frame all you need to do is repeat the previous steps, changing the colour you select from the Canvas Extension Colour menu.
Try creating a black line of around 5 pixels, followed by a white line of 20 pixels and finally add another black, 5 pixel line to the outside.
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