Making a custom border is as simple as painting it. All you must do is paint a sample border with a solid color where the digits will go, and the counter will use the components of your image to generate the border. Here's how it works.
Paint the border. Leave a solid area in the middle where you want the digits to go. If your border has a repeating pattern in it, you need to include an integral number of them in the sample border. Save this file as a gif in your web data directory (the same directory where the counter control file is). Here's an example:
The counter will take this sample border apart by looking at the solid section in the middle, giving it the corner and edge patterns to use:
Next, it will repeat the edges enough to match the size of the digits. Note that it doesn't scale anything, so if the digits aren't an exact multiple of your edge pattern, it won't line up at the corners.
Finally, it merges the digits, corners, and repeated edges to make the final image: