Photoshop Tutorials: Fill A Photo With Photos
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Step 3: Open A New 100 px x 100 px Document
Now that we have our cropped and resized images, we're going to create our pattern out of them. Go up to the File menu at the top of the screen and select New..., or use the keyboard shortcut Ctrl+N (Win) / Command+N (Mac). Either way brings up Photoshop's New Document dialog box. We want to create a new document that's twice as wide and twice as high as the two photos we just cropped, so since mine were cropped to 50x50 pixels, I'm going to enter 100 pixels for my document's width and 100 pixels for my document's height:

Also, make sure the Resolution for your new document is the same at it is for your photos. Select one of your photos and go up to the Image menu at the top of the screen, then select Image Size, which will bring up the Image Size dialog box. Check the value listed beside "Resolution". It will most likely say either 72, 240 or 300. Make sure your new Photoshop document is set to that same resolution value, which is located directly beneath the "Height" value.
Click OK in the top right corner of the New Document dialog box to exit out of it and have Photoshop create your new document.
Step 4: Drag Your Two Small Images Into The New Document
Grab your Move Tool from the Tools palette or by pressing V on your keyboard:

Then click inside the first photo with the Move Tool and drag it into the new document.

Click inside the new document and drag the image into the top left corner. It should snap into place:

Click inside the new document once again, and this time hold down your Alt (Win) / Option (Mac) key and drag the image into the bottom right corner. By holding down Alt/Option, rather than moving the original image, you'll be creating a copy of it as you drag and you'll end up with two copies of the image, one in the top left corner and one in the bottom right corner:

Now let's do the same thing with the second image. First, click inside of it with the Move Tool and drag it into the new document:

Then click inside the new document and drag the image into the top right corner:

Finally, hold down your Alt (Win) / Option (Mac) key once again and drag the image into the bottom left corner, making a copy of it as you drag thanks to the Alt/Option key:
