This post led to me exploring further Indesign/BookBaby.
I have not used InDesign (I am on Linux) and I was unaware of BookBaby before reading this post.
This is a learning exercise for me. Since you have no takers I will chip in with some ideas.
On first reading my understanding is that BookBaby receives PDF as input from authors.
https://www.bookbaby.com/video-tutorials-pdfsThis tutorial suggests exporting PDF/X-4 and this is an option available in Scribus.
Thus, to attempt to answer your question, one option open is to create your content using Scribus and upload to BookBaby the interim layout as exported PDF.
However, I rather suspect from reading here
https://www.bookbaby.com/book-formatting/that BookBaby might prefer to tweak the interim content (although probably for a fee).
Therefore, I would consider taking one step back and writing the content in raw markdown (using an editor such as Atom
https://atom.io/ for text and images) and use pandoc
https://pandoc.org/ to generate your PDF. I recommend the added package Markdown-Preview-Enhanced.
https://atom.io/packages/markdown-preview-enhanced. There are other Atom packages which can be installed to improve workflow.
To place one of your photo images in markdown you would use simply ..
@import ("/path/to/your/photo.jpg").
Indeed you can also @import micro markdown sections of your document.
A master page could be a string of @import commands.
This approach bypasses both InDesign and Scribus although the output from pandoc filters can be imported into either layout engine.
On a separate topic I have now read about importing Adobe *.idml into Scribus.
I'm not sure if that step is needed since the general idea is to avoid paying for Adobe product (at least until you can afford it from author's earnings). However, from reading around I do see now how it might be possible to sync *.idml with *.sla formats. Currently Scribus does not export *.idml.