grNadpa, if I might paraphrase Quoth to answer your question: use reflow.
If you need help doing that, try their solution if it suits you (eg: if you use Word) but there are other ways.
Fixed layout needs epub3 and is going to significantly reduce the number of devices on which the book can be viewed ... but this may be effectively true for your book anyway. If it is graphics heavy and those graphics need to be shown at a certain minimum size to be useful then perhaps you are already limited to larger tablets?
Do you have a 6" e-ink reader on which to experiment? Put a few of your images in a reflow epub (epub2 file) and load it up to see if it's useful/useable. If so, go with reflow to maximise where the book can be viewed. If not ... you could alter you book layout to better suit reflow, but if that doesn't suit I don't have the experience to help much. I'd normally go with pdf for fixed layout because of its almost universal support (even most e-readers will view pdf albeit awkward to use that way) but pdf would limit where/how you could sell the result.
In short: reflow is going to be the most flexible option if you can get it to work for your book.