To start off, you're going to need certain things. First, you need a computer, but since you're reading this we'll take that as read. Installed on your computer you'll need some type of image editing software. For the purposes of this tutorial I'm using the popular Adobe Photoshop Elements 5.0, but any recent version of Elements will do, as will Paint Shop Pro, PhotoImpact, GIMP, or any editing program that includes layers and text effects. You will also need a photo printer, some A4 glossy photo paper, some standard non-photo printing paper, and some sort of paper adhesive such as Pritt Stick.
Obviously the first task is to pick an appropriate photo to use as the frontispiece of your card. I've chosen a nice snowy scene with no religious overtones, but you can use whatever you like. For the method I'm using here a vertical (portrait) format picture is necessary, but you can adapt this tutorial to make horizontal (landscape) format cards as well.
The first thing we want to do to it is make our picture the right size to fit on our card. We're going to make the card out of a folded sheet of A4 photo printing paper, so our picture needs to be half the size of A4. A sheet of A4 paper measures 21cm wide by 29.7cm high, so our image needs to be 21cm high by 14.9cm wide. Since the original ratio of the picture probably doesn't match the standard proportions of printing paper, we have to do this in two stages. First, click on the Image menu, move to Resize and select Image Size. In the dialogue box, check the Resample Image box, then enter the width, 14.9cm or 149mm. You'll see that the height of the picture will change as well. If the new figure is larger than 21cm, click OK and go on to the next step. If it is less than 21cm, then delete the height and type in 21cm. This should change the width to be greater than 14.9cm. Click OK and go on to the next step.
Next, click on the Image menu, move to Resize, and select Canvas size. In the dialogue box, edit the dimensions of the canvas to be 21cm x 14.9cm, then click OK. You'll probably see a warning message that you are about to trim off some of your image. Click OK to continue.