Photoshop Tutorials: Fill A Photo With Photos
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Step 2: Crop The Smaller Photos
We're going to crop the smaller photos next, and we'll be cropping them into a square, just like we did with the main photo. Rather than using the Rectangular Marquee Tool though, let's use the Crop Tool, since we're going to be cropping these photos down to a specific size. Now, the actual size that you crop them to is going to depend on the size of your main photo and also on how many smaller photos you want to use to fill your main photo with, so you may need to experiment a little with this.
Grab your Crop tool from the Tools palette, or press C on your keyboard to select it:

With the Crop Tool selected, go up to the Options Bar at the top of the screen and enter in a value of 50 px (pixels) for both the width and height:

Again, you may need to experiment a little with the sizes depending on the size of your main image.
Now that we have our width and height entered for the Crop Tool, I'm going to drag a selection around the head of the child in the first photo. No need to hold down my Shift key this time to constrain my selection to a perfect square since we've already specified a width and height in the Options Bar. If you need to reposition your selection though as you're dragging it out, hold down the spacebar once again to move the selection around on the screen, then release the spacebar to continue dragging the selection:

You can use the handles in any of the four corners to resize your selection after you release your mouse button. You can also reposition the selection by clicking inside of it and dragging it around with your mouse. When you're happy with your selection, press Enter (Win) / Return (Mac) to accept it, and Photoshop will not only crop the image around your selection, it will also resize it to 50x50 pixels (or whatever size you entered for the width and height in the Options Bar):

Do the same thing with the second photo, dragging a selection around the person's head with the Crop Tool, resizing and repositioning it as needed, and then pressing Enter (Win) / Return (Mac) to accept the selection and have Photoshop crop and resize it for you. Here's my selection around the young girl in the second photo:

And after pressing Enter/Return, here is my cropped, 50x50 pixel image:
