Just wanted to chime in with a few thoughts...
First of all, to answer bspimas' questions - yes, you can definitely use your own scans, images, text...anything! The only "requirement" is that the "blank canvas" you use are the pages of one of our Digital Vintage Books. But you can add absolutely ANYTHING to those pages!
If you look at billizetti's great samples, you'll see that sometimes he covered the entire pages with his own art. I encourage you not to feel restricted by what already exists on the pages. Of course, one of the coolest things about altering books - be they digital or "physical" - is that you can work with what is already there. BUT you shouldn't let that hinder you. The pages you get in your Digital Altered Book download are really just blank canvases -- and what you do with them is completely up to you.
Sometimes, there may be elements on the page that are inspiring to you somehow -- that is, if you want to keep a word, or a sentence, or an image. Sometimes you may want to feature prominently what exists on the page, and just add a few embellishments to it. Or sometimes it may be just the TITLE of the book that inspires your theme - but then you basically create your own art on the pages, with little integration of what already exists on the book's pages. And of course, there are a million variations in between.
I think perhaps with the digital version, you might feel more like you are "supposed" to keep certain existing elements intact (whereas in a physical book, it's more comfortable to just go crazy and cover stuff up). The bottom line is that, just as in physical altered books, there are no rules, here. You're starting with some "blank canvases" and you can do anything you want to with them.
It may be easier to think of creating a series of 8 pieces of art on a certain theme (plus a front and back cover) rather than thinking of creating a "book". For example, you could use "The Bookshelf for Boys and Girls" as an interesting scrapbook album for photos of children in your family, and create a page for each child with pictures, and words and embellishments that relate to them. You could use the book "Imperial Purple" to be a series of color-study collages, each one in a different color.
And, just as with a physical altered book, you could start working in one theme and end up in a totally different direction, and that's great, too.
Does this help at all?