Is there any control over what image roll20 selects for each page in the page select toolbar?

Hello, Fairly new to roll20, and i have made about 4 pages of maps, the first two i just used a generic background, and that shows up as the icon when you go to the page select toolbar, however, the last two maps i made, i decided to get fancy and started adding separate tokens for buildings and trees and such so that they could be moved around to randomize the maps easier. I like this method better, BUT, when you open the page select toolbar, the icon sort of gets randomized. Currently, one is a small map piece I added, and the other is a tree. Both of those are in the background layer, but they overode the original background map that was there first. I can't find any way to control what icon shows up, other than only using a single background image, which requires me to make a jpeg of the map in a jpeg editor before uploading. Also, my maps are pretty big in size, and the beauty of having background "tokens" is that you can minimize the amount of space you take up with uploading maps, and you can make unlimited maps using the same tokens for walls and trees and obstacles and such. Its not a huge deal, because I name the pages, but its funny to open the page tool bar and get such a funky looking image for the map, something that doesnt really reflect at all what the map looks like. Its not a big deal now because i only have 4 maps, but someday if i have 100 say, then i would need to open each one to find the one i want, rather than just scrolling thru the toolbar and looking at the icon images. At first I thought it used the first image you placed in the background. But that didn't turn out to be true, so I thought maybe it was the last image you placed in the background, but thats not true either. It seems sort of random. Sorry this was so long, I had trouble wording the problem and explaining why its a problem for me specifically.

I believe it uses whichever image is scaled the largest on your map. So for like your tree one, it is the largest token/map object on the page.

I believe William is correct that it is whatever the largest image on the page is. This answer from Steve K. would appear to confirm it.

David, what William said is correct. It's the largest art asset on the page. I know some GMs who take advantage of that behavior to put a large label image that's expanded to be the largest image, but dropped to the GM layer and behind the actual maps and tokens in order to make the page images easier to navigate. edit: ninja'd by Sarah and myself :P

Ok great, yes, now I'm getting it, on those particular maps, all the images were 6 x 6 inches, so i guess since all the biggest image pics were the same, it was just sort of picking one. I have resized them and now its working great, the image I want is the largest and the one that shows up on the tool bar. Thanks for the help, again.