Thank you so so much for that advice! I guess my mind was stuck on epub!
What I ended up doing is splitting the double page images with paint.net, then using irfanview to convert to grayscale, resize, and add a final centering border using canvas size. Then I used winrar to pack the images, renamed it to .cbr instead of .rar, and then transferred the files to my SD card. The files are 3x as big as before, but it's really negligible.
Thanks so much.