Hi there,
By behind I assume you mean over top an image.
There are different ways, the easiest way is mentioned above.
Following the same principle you can create a new file with a transparent background. Design your watermark etc and save the file as a .jpg or tiff.
While your watermark is still open in Photoshop open the image you want to apply the watermark on.
1) Go back to your watermark image and 'select all' or simply Ctrl + A.
2) Then press Ctrl + C
3) Go to your image file now and paste the watermark on top of the image by pressing Ctrl + V.
A new layer is then created and you can change the opacity to whatever you like to adjust the visibility of the watermark.
4) Save the file as a psd and keep this file for later use.
5) If you want a web friendly jpg then open the psd and save a jpg version.
Hope that helps!
-SC