For my customers who don't have Adobe Photoshop (either the full version or Photoshop Elements) I recommend a free program that can be downloaded. It's a software called Paint.NET and can be downloaded from http://www.getpaint.net/. (I don't have any affiliation with this company or its designers.)
You need to have your transparent watermark file [this has a .png file extension] created and saved prior to beginning this process. You also need to download and install the program.
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To use a watermark in PAINT.NET:
Open Paint.NET.
Open your card photo that you want to watermark.
In the file menu across the type, click on LAYERS, IMPORT FROM FILE and browse to the folder that your watermarks are saved in. [I provide both a light and a dark version of the watermarks I create for my customers] Double click on the version you want based on whether light would look better or dark would look better against your image. This will layer your watermark over the card photo. If your watermark is larger than the card photo, the canvas size will be automatically enlarged to accommodate it and you'll see extra white area, probably to the right of your initial image.
To resize your watermark so it fits over your card photo, click on the .png image while your pointer is one of the arrows with the + sign next to it from the from the vertical toolbar on the left. You’ll see a border of what look like marching ants around the perimeter. Hold down the SHIFT key and click and drag one of the corners inward. Holding the shift key allows the image to keep its shape while being sized down which eliminates the possibility of distortion.
When the watermark is the size you want it to be, click on the .png and you can move it around to layer over the card photo wherever you would like it to be. If you want the "marching ants" to go away, click CTRL+D (unselect).
Now you'll delete the excess "canvas" if there was any from layering your .png file. You can do this by choosing a different tool from the vertical toolbar on the left. You want to select the dotted line square in the upper left. Draw a box around what you want to keep in your final image. Count eight icons in from the left to the icon to the immediate right of the paste icon. If you hover over it without clicking, it should show a label that says CROP TO SELECTION. If you didn’t get it all cropped closely enough, you can repeat the process until you get it where you’d like it. If you crop too much, you can type CTRL+Z (undo) and it will undo the last step you took. You can continue to undo until you get back to the last point where you liked how it was.
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I hope this was helpful!