That is so infuriatingly sad: impossible!
If they had only empowered the print module to create templates for the book module...!
If you want to use JPG as output for the book anyhow and have no text, you could go for the print module instead. There you have the option to have an inner stroke, serving as clean border, width & colour of your choice. Plus you specify the page size according to your wishes, not just the limited Blurb offering.
Text is gone as soon as you choose "Custom package" i.o. "Single image / contact sheet" - that is so frustrating, because "caption" would do, as it allows it to put sufficient text into it. Albeit not with the same text formatting options as in book module text boxes.
I am just trying to get a nice PDF with several images per page to display on an iPad...
Doing it from the book module leaves those ugly grey borders due to non-match of size.
Doing it from the print module robs me of text, unless I want the boring grid layout contact-sheet-style.
Doing it from the slide module works best, but limits to 1 image per size.
Unfortunately, the Book module is plagued with limitations and shortfalls in capabilities. This is but one. While I am well aware that ... "you have to start somewhere" ... I, for one, expected so much more from the folks that pioneered apps like PageMaker and InDesign ... of course I don't expect the feature laiden aspects of those apps ... but I also don't think the Book module should be as limited and confined as it is in it's current state ... It is the way it is, intentionally by design and choice of the folks in charge of it's development ... no other reason ... they got it wrong and won't admit it expecting users to just accept this half-baked one-trick pony as is .... We deserve better ...