If you had a link to what you've seen it would help.
But in general that could be (1) the image is a background image (2) the content 'above' the image is in a div (layer) which is set in zorder (z index) 'above' the image in question.
To get it to look translucent... there are GIF images whereby you get an on/off alpha... You can use these to dither a pattern which looks like a filled area of approx 50% 'transparency' (every other line horizontally, vertically, or every other pixel in a checkerboard pattern). If you were to use for example a tiled GIF image which is a 'checkerboard' dither of medium grey in a layer 'above' other content, it would appear to be a translucent window of grey (on closer inspection you'd see the dither).
The PNG image format also supports transparency, and actually supports alpha transparency (like photoshop layer opacity/masking/etc.). Problem is none of the browsers ever fully/properly supported PNG rendering -- you can technically save a PNG as 8-24 bit with alpha -- much more flexible than GIF. It compresses via color reduction and then some basic file compression (like GIF), so its really too bad it was never fully implemented.
The only 'hack' method I could think of that could get your true translucency/alpha channel would be to bring PNG8/24 images exported from Macromedia Fireworks (very important as ones from Photoshop/IR do not work the same way in this method) and put them in a Flash movie. Then -- only for Internet Explorer and I believe only on PC -- you can embed the flash movie on your page with the background property set to 'transparent'.
What happens is, the flash movie in a layer on the page will have no solid bounding background... and if there are any alpha images/soft edges/etc. in the movie, it will render right on top of your page.
Of course this limits you to pretty much IE/PC users only (any version of flash from 4 up). A significant number of people -- but not everyone.
Hope this helps,
Neil
alinear.net