You can create interesting effects by creating custom preview images for the albums in your gallery. Follow these steps:
Create the gallery however you wish (see here: Tips-MergingAlbums). For our example, we are going to replace the entire contents of the album preview with an image.
As such, you want to make sure that the slideshow uses "Albums / Preview Style = Fill" and "Albums / Preview Scale = None". This will allow us to recreate the preview from scratch.
SSP/LR takes care of lots of things. Due to its auto-fit nature, there's no good way to figure out what your preview image dimensions should be from the SSP/LR settings. This is left as an exercise to the reader. The dimensions you measure are a good starting point.
You have lots of options here. Create the images however you want. The only thing you want to make sure is that the dimensions are proper. This isn't an exact science. It depends highly on the options you chose while creating the slideshow.
You're probably going to want to copy the images to the slideshow directory. I would suggest creating a sub-directory called 'previews'. Once that directory is created, copy all of the custom images there.
images.xml is the file you modified to create the gallery. Open it with a plain-text editor (like Notepad for Windows). Actually, if you're reading this you probably know that by now and will opt for a syntax-highlighter editor anyway (I use Notepad++).
You need to change the tn parameter for each <album> element in your gallery. Just as an exmaple, lets say my album1 directory contained my "Animal" photographs and I created "animal_preview.jpg". I would change this:
<album id="Animals" lgpath="album1/images/" tnpath="album1/thumb/" popuppath="album1/popup/" fspath="album1/fs/" title="Animals" description="Animals" tn="album1/thumb/CIMG5699.jpg" audio="" audiocaption="">
to this:
<album id="Animals" lgpath="album1/images/" tnpath="album1/thumb/" popuppath="album1/popup/" fspath="album1/fs/" title="Animals" description="Animals" tn="previews/animal_preview.jpg" audio="" audiocaption="">
Modify each <album> element and you're all done!