If you ever find yourself needing more canvas space around your image, you can always go up to the Image menu at the top of the screen, select Canvas Size, and then add more space using the Canvas Size dialog box, but that's the long, clunky way of doing it. There's an easier way, using Photoshop's Crop Tool.
I have an image open here:
And I want to add some extra canvas space around the image. To do that, I'm going to grab my Crop Tool from the Tools palette:
I could also press C on my keyboard to quickly select it. Then, with the Crop Tool selected, I'm going to drag a selection around the entire image, starting just outside the top left corner and dragging all the way down to just outside the bottom right corner. You'll see the "marching ants" around the entire image:
When you release your mouse, you'll see a transform box and handles around the image, as if you had selected it with the Free Transform command. Hold down Shift+Alt (Win) / Shift+Option (Mac) and drag out any of the corner handles to add space around the image. Holding Shift constrains the width and height proportions, and holding Alt/Option resizes the selection from the center. If you need to resize just the left and right side, drag out one of the side handles while holding Alt (Win) / Option (Mac). To resize the top and bottom, drag out either the top or bottom handle, again while holding Alt/Option:
When you're happy with the amount of space you've added around the sides, press Enter (Win) / Return (Mac) to accept the transformation and Photoshop will turn that extra space into canvas space, filling it with white or whatever happens to be your background color:
And that's how you can quickly add more canvas space to an image at any point using Photoshop's Crop Tool!
source : www.photoshopessentials.com