Make a custom / composite image of vignettes to serve as a unique banner. This example, Travels illustrates the effect that I like, giving the viewer a hint of highlights in an album.
How-to-do, a bit time consuming but worth it for special albums.
My "banners" are commonly 900x240 px (it's bigger in the example). I use The GIMP to create such images but other editors can be used. Here's the method that I use:
Create a blank image 900x240 and fill it with choice of background color. I use black.
For vignette-like effects, from part of an image to be featured in the banner, select an area using a selection tool, e.g., an ellipse, using feathered edges as a tool property. I like a large value like "100." Copy this to the clip board. This works best if the underlying layer for an image is null / transparent, so that "cruft" doesn't drag along from the background and muddy the fuzzy fringe of a vignette.
Return to the master banner image and paste the selection therein, making it a layer to manipulate. Resize the pasted selection, manipulating that layer content, to fit nicely / proportionately in blank space of the banner.
Do this repeatedly with selections from other images, placing each selection in its own layer. In the resizing step, it can help to work on the resizing in another window with a transparent background because a selection from a large image may be difficult to scale in on a small, e.g., 900x240 canvass.
Manipulate the layers individually to resize and position so that there is a pleasing relationship between each vignette.
Save the banner image and copy it into the folder jAlbum for which you wish the image to serve as the banner image. As pointed out in other tips, designate this the folder image by right-clicking on it and selecting "Use as folder thumbnail." I also like to mark these images as excluded so they do not show up as thumbnails in the folder and in a slide show. This can also be done at the top-level of an album by in the "master folder" containing all folders for sub-albums.