This tutorial will teach you how to make very nice borders for images using a Color Halftone filter.

Open any picture you want to apply that effect to. If you open an image and it doesn't let you make new layers or channels, just make a new file and paste the image into there.

Open up Channels Palette and make a new Channel by pressing on the new channel icon(). Name it Alpha 1. Press D to set up the colors to standard (BG: White; FG: Black).

Using the Rectangular Marquee Tool(), drawa rectangle around the area you do not want to be affected by the filter. Press Ctrl+Shift+I to invert the selection.

Delete part of the layer by hitting Delete) button. Make sure your colors are set up to standard.

Now run the Color Halftone Filter (Filter -> Pixelate -> Color Halftone). Leave the default settings. The only thing you might want to change is the maximum radius of circles. It might take few tries to achieve your desired result.

Now Ctrl+Click on Alpha 1 channel to select all the white areas. Switch to layers and hit press delete to erase unwanted parts of the picture. You should get something similar to the picture below.