Yeah, it is kind of frustrating, but the good news is, it isn't as bad as you think it is.
Although AC3D does not (re)place your backgrounds when you reopen a file, it CAN save the setup so that you need only "load" the entire set of backgrounds at once, rather than reloading and repositioning all four viewports each time.
1) After you set up backgrounds in all four viewports, save the entire background configuration in the menu:
Tools > Save background image configuration. Give the file the same name as your AC3D project, and, possibly save it in the same location, so you'll be able to find it later. It has its own suffix (.acp, instead of .ac for the model itself).
2) After you save the background config, and then save, close and re-open your AC3D project, go to
Tools > Load background image configuration. Select the .acp file and they'll all be placed, just where you last left them, in a jiffy.