I'll have to look into that in more detail tomorrow (it's 2:15 am here and I'm beginning to fall asleep).
I've considered just putting the little red submenu icons into progmem as monochrome bitmaps, since the rest of the header will always stay the same... but I'm worried that they will look blocky, as many bitmaps tend to do when you reduce them to monochrome. As they are now, they are full-color, anti-aliased and smooth looking.
It may be slightly unorthodox in programming terms to load an entire header image for every submenu, and very similar looking ones too, but I just want the user interface to look its absolute best.
In web development, where I've got my programming roots, you can easily set the background of a full color antialiased icon image as transparent and then make it blend in smoothly with its surroundings, for example a header image that then indeed only needs to be loaded once. And you can save a bit of memory and bandwidth that way, although in the age of high speed Internet and decent system resources of the average desktop computer or mobile device, it's not that relevant anymore in the first place and is mostly done for practical reasons.
As far as I'm aware, transparent backgrounds do not exist for full color bitmaps on Adafruit displays, do they?