How do I make a look through image?

I want this 'look through image' (really a small gray rectangle) to be shown in a layer over some larger (more detailed) image.
On the web the effect would be that the detailed image underneath shines through the gray image on top. In other words: How can I get a degree of transparency?

Put the small gray rectangle on a layer above the background image. (Do you know how to do this?) Adjust the opacity of the top layer by using the opacity adjustment in the layers palette.

Thanks, Mark! Your solution works out fine, although I had some trouble to add text on top that layer. The grey image was supposed to be only a background for the site's title. At first the title text received low opacity as well, but later I got it to work properly. See <http://www.radiuswebdesign.nl> for results.
Still I am curious if you or anyone else, knows a way to incorporate low opacity into an image itself. ???

If the image consists just of the Background layer, you cannot lower its opacity since the Background layer is locked. But if you rename the layer to anything else you can then change the opacity.

Was that your question, or something else? What are you trying to accomplish?

AND, when you're image editting is complete, you can flatten the image, if you choose, getting you back to just a single layer .

All right. See what you mean. But where do I go after renaming that layer to actually change opacity somewhere? Tried all buttons here (cannot translate): <http://www.radiuswebdesign.nl/pix/elements.gif>, but no opacity is shown in a test image, nor saved, nor later displayed in a test page ....

I will give this one more try, tomorrow, else I'll stick to Marks solution. Thanks for helping, anyway!

I'm trying something new here, so it might not work! To change the Opacity, look at the upper right part of the Layer Palette. Now let's see if my picture shows up. (Don't be surprised if it doesn't; I won't be!)

href="http://www.pixentral.com/show.php?picture=1tXfWFmwiePEAw7VWT3yItSwUPrK0"/>

Woohoo! I did it! Of course you're going to have to look very closely or you won't see the line I'm talking about.

Live link:

<http://www.pixentral.com/show.php?picture=1tXfWFmwiePEAw7VWT3yItSwUPrK0>

Show off. :)