One day Llama Louie (that's not his real name; we're disguising it because he is very shy) came into our shop, muttering and cursing.  "I can't find a way to do it," he grumbled.  "To do what?" I asked.  "To get my logo in the bottom right corner of every picture, no matter what size it is or how dark the image is, so I can make an action and use it in a batch process.  I need to use it for images from 4x6 to 11x14 without having to adjust each one."

"Have you done any research?" I asked him.  "Yes," he said; "I've Googled and Yahooed and searched the Internet exhaustively.  People have been trying to do this for at least 10 years, and they've never gotten it right.  Either their method works for small images but not large ones, or for large ones but not small ones.  Or it doesn't always place the logo where I want it, flush against the bottom right corner.  And sometimes you can't see the logo because the image is nearly black where it's supposed to be, but if you try to use color, it looks bad for other images."

"Well, you've spotted the three major obstacles," I told him.  "It must be your Llama training."  And then I outlined the problems that have to be solved, in a manner that's suitable for a Photoshop Action:

1. How to size the logo so it's exactly right for images of arbitrary dimension;
2. How to move the logo precisely to the lower right corner of the image;
3. How to make sure the logo is clearly but subtly visible, no matter how dark the image's background.

Next I told him that -- briefly -- here's how we'll do it:

1. We're going to "place" the logo and size it with Ruler Units set to Percent;
2. We're going to link the Background image and Logo image and then Align the layers Bottom and Right;
3. We're going to use a logo that's 50% Gray with a Bevel and Emboss style, and blend it with Hard Light.

Well, I wish you could have seen the look on his face.  Awe, and gratitude, and worship.  I was embarrassed.

But then we set to work.  Following the ideas in Detecting the Invisible, we created the following Logo:

This is a PSD file, 20 in. x 20 in., one layer, with Llama Louie's Logo (the two L's) in 50% Gray with Bevel and Emboss on an empty background.  He named it My_Watermark.psd.  Important: the Blend Mode for the single layer is set at Normal (that's why you can see it).  "Now remember," I said, "we're going to wind up using Hard Light for the Blend Mode."  "But in Detecting the Invisible," Llama Louie muttered, "you said we should use Overlay mode."  ("Hmm...", I thought.  I didn't know he'd even read the issue.)  "That was then, Louie, and this is now," I said.  "Things change."

We had to do one other thing.  In Step 2 above, the Align Layers command will adjust the only VISIBLE parts of two layers, not necessarily their right and bottom image borders.  So, to keep the logo away a little distance away and a little distance up from the lower-right corner, I had Llama Louie put a visible but really small dot of gray color in the lower-right corner of his logo.  It's so small you can't see it here -- but Photoshop can.

Next I explained our strategy.  Normally the Ruler Units are set to Inches, and sometimes to Points or Pixels.  But in this case, where we want one image size to be based on another image size, the Percent value is best.  Llama Louie nodded, his eyes wide.  Also, to position the resized logo, I said, we would link the two layers (Background and Logo), and then use the Layers > Align command.  "Never heard of it," he muttered.  "That's all right," I reassured him; "nobody else has, either."

Then, with Llama Louie hunched over the keyboard, I told him to start by opening an image.  He selected, not surprisingly, this one:

"Good choice," I told him; "see the dark area in the lower right corner?  Watch what we do now."  And here's what I told him to do.

1. Start an new action, and name it "Add Logo";

2. Select Edit > Preferences > Units & Rulers, and then set "Rulers" to "percent";

3. Select File > Place, then navigate to and double-click on "My_Watermark.psd";

4. When the logo layer becomes visible with its Transform markings, select a corner (not a side) and while holding Shift + Alt, drag that corner until the logo looks about right for the size of the image;

5. Before you let go you should see something at the top that looks something like this:

6. Around 12% is good, because that's about an 8 to 1 ratio, good for almost any size image;

7. Now hit OK and proceed;

8. Go to the Layers Palette and select both layers;

9. From the Layer menu, select Layer > Link Layers;

10. From the Layer menu, select Layer > Align > Bottom Edges;

11. From the Layer menu, select Layer > Align > Right Edges;

12. From the Layer menu, select Layer > Unlink Layers;

13. Go to the Layers Palette, select the Logo layer, and set the Blend Mode to Hard Light;

14. Finally, select Edit > Preferences > Units & Rulers, and set "Rulers" back to "inches" (or whatever you normally use).

15. Stop recording the action.

When he was done, here's what we got:

"You see?" I said.  "Perfectly sized, perfectly placed, and nicely visible in that dark corner."  Then I had him change the Blend Mode to Overlay.  This is what we got:

It's there, but you really can't see it.  So Hard Light is better than Overlay in dark areas.  But just to be sure of everything, I had Llama Louie delete the Logo layer, rotate the image of himself through 90° clockwise, flip it vertical, and apply his new  "Add Logo" action.  This is what he got:

Again, perfectly sized, placed, and visible -- but not too pushy.  Clearly Hard Light is okay for the lighter regions, as well.

"A final note", I explained to Llama Louie.  "Once you get your logo just where you want it, be sure to flatten the image before you apply any image size changes.  For some reason, Photoshop doesn't crop both layers the same if they haven't been merged."  Llama Louie nodded, his eyes big with wonder.  "I can do the rest myself," he announced, as he walked out of the shop.