I found the solution to something I posted about a while ago.
I have a number of pictures on my pages that I wanted to automatically "dim" and have text (ie: "Click to place your 120 x 60 ad here") at set times. I thought I was going to have to settle for making large kb three-layer animated gifs (transparent layer/partial opacity copy of original picture with text layer/transparent layer) with calculated delays in the "firing" of the layers, so the pictures would appear to change (dim with lettering) in sequence around the page. That would have ballooned my bandwidth because it was almost like having two of each of the original images in Kb size. The reason for this was that, while I could make an independent "dimmed" gif image by combining the original picture with a partial opacity white layer, I couldn't seem to make an independent partially transparent white image with text. When I tried to combine (layer down) a partial opacity white image with a transparent layer, it would either be totally transparent or totally white (depending on the opacity setting) after saving. There was a threshold that, crossed, threw the opacity to either 0% or 100%depending on which side of the threshold the opacity setting was.
I got back into the Gimp online manual and found the problem. Gif does not support an independent partially transparent image. Png does! Aha! I created a partially transparent white image.png and placed it over a picture on a page. It worked. The size of the "cover" image is 282 bytes! It shouldn't be much larger with the text and the two additional "timed" transparent layers.
Bottom line is that you can build a "mixed" gif by combining two images with the top layer (image) having partial opacity and then combine the layers to give the overall gif the transparent layer effect, but it seems you can't create a stand-alone partially transparent gif. Now I'm happy. I'll be able to slash my bandwidth by a massive amount.