It looks like there is a lot of documentation on the Debian wiki about creating your own version of the Debian installer, with multiple routes available depending on what you want to do.
You could try poking around at the graphical installer package and then following the instructions to build an ISO:
https://wiki.debian.org/DebianInstaller/GUIBuild
(Edit: that repo seems to be broken. Maybe this one?
https://wiki.debian.org/DebianInstaller/CheckOut
Edit again: I played with this a little bit and didn't make much progress, I'll try again in the morning. I suspect I'll need to do this as well at some point, might as well figure it out now.)
You could also try to mount a prebuilt install image after downloading it, searching for the location of image files, and substituting your own, then make it back into an ISO.
https://wiki.debian.org/DebianInstaller/Modify/CD
Changing the wallpaper should be trivial to automate, you just have to find the right script to add a command to, and the command depends on the desktop environment you are installing. Or, if you can find the default wallpaper, replace it with yours, using the same name and file type.
If you post more information on where you're getting stuck, I'll do my best to answer any questions. However, I have not actually tried any of this before. Just googling around I don't see any easier way to do it.
