how to split epub full of .png files

I downloaded a epub that is a combination of sanskrit and text and each page is a .png file and it is quite large so I want to split it into 4 chapters by selecting 4 blocks of png files. I have the split plug in installed.

I think I need to build a toc to do this correctly but can't seem to get started on how to do it.

Any hints would be appreciated.

Thanks.