Really, he could just have any lines past the main known ones to be considered as an image. The line format could be the same, with the addition of a filename field on the end. Like so:
8 48 584 480 brothelback2.jpg
You could then add as many images as desired like that, each on their own line. If necessary, they could be prefixed with a number indicating how many such images to process (like is done for processing traits and such in the other files).
The images would preferably be added in the background behind everything else (placed before everything else if that's needed to get the order correct) in the order that they're given in the file. That way, they could be used as background images for text and other controls.
I doubt I'd actually use that much myself, but it would still be quite handy.
EDIT: a more detailed example, using BrothelScreen.txt as an example:
16 20 768 576 1 WINDOW - XPos, YPos, WIDTH, HEIGHT, BORDERSIZE
600 258 160 32 Girl Management BUTTON - XPos, YPos, WIDTH, HEIGHT
600 298 160 32 Staff Management BUTTON - XPos, YPos, WIDTH, HEIGHT
600 338 160 32 Upgrades BUTTON - XPos, YPos, WIDTH, HEIGHT
600 398 160 32 Dungeon BUTTON - XPos, YPos, WIDTH, HEIGHT
600 458 160 32 Visit town BUTTON - XPos, YPos, WIDTH, HEIGHT
184 536 160 32 Save BUTTON - XPos, YPos, WIDTH, HEIGHT
600 536 160 32 Next Day BUTTON - XPos, YPos, WIDTH, HEIGHT
432 536 160 32 Turn summary BUTTON - XPos, YPos, WIDTH, HEIGHT
8 536 160 32 Quit BUTTON - XPos, YPos, WIDTH, HEIGHT
600 208 72 32 Prev BUTTON - XPos, YPos, WIDTH, HEIGHT
688 208 72 32 Next BUTTON - XPos, YPos, WIDTH, HEIGHT
8 48 584 480 Image - x,y,width,height
600 48 160 152 10 Brothel Details text - xpos, ypos, width, height, fontsize
16 8 900 64 20 Brothel Name text - xpos, ypos, width, height, fontsize
3
8 48 584 480 brothelback2.jpg
8 248 584 480 brothelback3.jpg
8 448 584 480 brothelback4.jpg
Another interface request:
Give us the ability to change the height of listbox entries, or alternately necno could simply change the size himself; they're currently too tall really, wasting space which could used to display more entries.