To create our stamp we need a document of appropriate size. Mine will be 400x400 and I will reduce it later. Fill the background black and then create a new layer: Layer-New-Layer. In it, draw a rectangle using the Rectangular Marquee Tool.

Fill the rectangle white using the Bucket Tool and select the Brush Tool.

Go to the brush settings and select the following:

Now you need to select the Pen Tool touch the four corners of the rectangle. Start on one side and go around it until you go back were you started to close the loop. Right click layer and choose Stroke and Brush.

You will get this shape and then you have to right-click again and select Delete Path.

Grab the Magic Wand and touch the white part to select it. Click on Edit-Copy and then Edit-Paste.

Open a picture of something you want to put in your stamp. I my case it will be her Majesty Queen Elizabeth of course. Use the move tool and drag the picture into your document. Now use Edit-Transform-Scale to make it fit in your stamp.

You now have a plain looking stamp but if you want to make it more realistic you can do this step: Make visible only the picture and stamp layers. Go to Layer-merge visible and then right click layer and choose blending Options. Then choose what you see in the following picture.

And there you have it: a very nice looking stamp.