Choose two images: one will be for your background and the other will the image you wish to make a stamp. In this example, the little girl image will be the stamp and the Valentine will be the background. You will not be able to make the stamp if your image size it too large. To reduce the image size, click "Image". Then click "Image Size".
Change the width and height. I randomly choose 800 pixels and 1200 pixels and had no problem continuing. Click "OK".
Next you will need to unlock the image you wish to make into a stamp. Do this by double clicking the "Background".
A box will appear that says "New Layer"; click "OK".
On the tool bar, locate and click the "polygonal Lasso Tool".
Click around the image you want to use as your stamp. Remember, to complete this step you will have to click right back to where you began your first click (a tiny circle will be there to guide you).
Next, Click "Edit"...
Click "Define Brush Preset". (This is where the problem would appear if your image was too large)
A window called "Brush Name" will pop up. Name your brush or just click "OK".
Now go to your background image.
Click the "Brush Tool" on the side bar; Click "Brush" on the top tool bar; scroll down to the last brush... there you will find the brush you just created. Click on this brush. Adjust the Master Diameter slider bar to make the stamp as small or as large as you desire.
Stamp your background as desired. Play around with the color, opacity and size to get the effect you like. You can stamp as few or as many as you like.
Additional effects can be done by clicking the "Brushes" tool at the top right of the screen. The "Scattering" effect will add the image many times randomly. ...for a look like this. Have fun and play around to get just the look you want.