Photoshop :: Best Way To Add A Visible / Transparent Watermark?

I add a watermark to all the photos I post to the web.  I've (for years) used a transparent .png file and it's worked fine.  Since starting to shoot with Canon Raw, I'm now doing the editing in CS5 for Windoze.  (W7 pro 640 bit) 
 
Since I've been unable to find a visible watermark feature in CS5, I've been dragging, then dropping the .png file on the edited image, placing it, then adjusting the opacity to a value of 5 or 6.  That works out ok. 
 
But when I drop the .png onto the edited file, its size is inconsistent.  Well, it's consistent in that it's about 150 percent of what it should be.  Both the edited file and the watermark image are 800 pixels wide.  However, at what seems random frequency, the dropped image is about 1200 pixels wide, necessitating alignment and resizing before placing. 
 
First, have I overlooked a visible watermark function?I know that's a loaded question, but if it's a method that should work, why is the dropped image inconsistent in its size?  Next, is it reasonable to create an action to overlay the edited image with the watermark?  I always use the same file, so it would seem like a reasonable thing.