Thank you for your advice and feedback on the image.
I am having a real game creating an action to add the watermark image to the bottom left hand corner 10 pixels from the left and bottom edges and then resizing to 640 pixels on the longest edge.
I want to select the images to be resized and watermarked, have them edited according to the above description and then automatically saved (with wm added to the original filename) and closed (regardless of whether they are landscape or portrait).
The only time I want to get involved is when I want to use a dark watermark instead of a light one.