"Classic": 4 sloped ramps', 4 flat ramps, 2 center tiles, 2 pillars = 12
"Half Height": 4 half-sloped ramps, 4 flat ramps on half slopes. = 8
12+8=20
Assymetry: *2 (that is, if center tiles and pillars are assymetric as well)
Winter: *2 (everything can have a winter graphic)
Front image: *2 (everything can have a front graphic)
20*2*2*2=160
What you forgot to include in your number were the doubled flat ramps with half height, winter graphics and pillars. And that's understandable, you'd need to be crazy to create 16 different graphics just for pillars, but they exist (actually, I think you can even have "full height" and "half height" pillars, which would be even more potential images)
I did not mean that as an argument opposing the idea, but rather as an reminder that the max number is not important as all. After all, the current image count is 80, yet the last bridge I made with both winter and frontimage used only 44 images, and the one before just 30, and is still complete and usable.
') I use "sloped ramps" as those where the way itself is sloped and "flat ramps" as those where the way is flat, but which are buildt on slopes