I've been using Photoshop to edit these files, but other image editors may work so long as they support IFF files being written with indexed colors and in a format the Amiga is happy with.
First off, find your source image. It doesn't have to be super simple, but the simpler the color palette, the better the end result will look. Here's a non-conspicuous picture of Ned Flanders I'm going to use:
Create a new image that's 640 pixels wide. The height is a bit irrelevant as I haven't seen any adverse effects of using a height larger than the given logo space outside of the banner scrolling. If you want a reference height, the old TVGBANNR.UV file is 640x160.
Next, you're going to want to set the pixel aspect ratio to a custom aspect ratio of 0.85. This helps you get an idea of how the image would really look on a TV screen given the Amiga is drawing the data at a wider resolution and then horizontally squeezing it.
Take your source image, and just paste it into the active document in Photoshop. It'll adjust the pixel aspect ratios for you so your image should come out relatively normal looking when viewing it with the aspect ratio correction on. Now, we need to reduce the color count. Select Image>Mode>Indexed Color.
We're going to set our image to 8 colors, but with a caveat.
The first 4 colors must be the following RGB values: 0, 0, 49 [dark blue]; 191, 191, 191 [grey]; 0, 0, 0 [black]; and 191, 191, 0 [mustard yellow]. If you try to use any colors past the 8th index will just wrap-around to the first, second, third, and so on indices. We can do this by clicking on Forced > Custom and setting the colors there:
Also important to note that the 8th color index becomes the color that's used to fill in the left/right hand sides of the image when it's drawn in the guide, as the image doesn't fully take up the display size.
You can play with the dithering settings a bit to make the image look better.
When you're done, let's save that image! File > Save As, and make sure you pick on your computer if you're using newer versions of Photoshop
Give it a good name...
...and make sure you save it in Amiga format.
Now, just copy the banner file to your Prevue/Banner directory replacing TVGBANNR.UV (optionally, you can back up the old banner):
If done right, your guide will start using the new banner. You shouldn't even have to restart the guide, as it appears to reload the banner each time the scroll cycles through.
It will look stretched out horizontally as that's what the Prevue software expects. If you feel so inclined, adjust your emulator's aspect ratios to account for this so everything is shrunk back to their normal width. But that's it!
*NOTE*: While you can replace the banner in 7.8.x versions of Prevue, the color palette is completely limited to that of the same colors used for displaying text ads here: https://prevueguide.com/wiki/UVSG_Satel ... and_Colors -- also, there doesn't appear to be a need to adjust pixel aspect ratio for those installs. The location of the banner in 7.8.x is also different: it's /Banner/TVGBANNER.UV in 9.0.4 and /PGWAWO.160 in 7.8.x -- when in doubt, you can always open BANNER.INI and see where it's pointing to!