In looking through the copy edit of Before the Big Bang (see the previous post) I noted with mild horror that I had forgotten to source an illustration.
It might come as a bit of a shock if you don't write non-fiction books, but generally it's down to the author to find the illustrations and (here's the nasty bit) pay for them if they need to be paid for.
The worst example of this was my book on the Victorian moving picture pioneer, Eadweard Muybridge. I could hardly write a book about a photographer without including a fair number of photographs, but the amount I was being quoted would have eaten up most of my advance.
Because this was a biography rather than a book about his pictures per se, I didn't need anywhere near as many photos as a true illustrated book, and I persuaded the sources (mostly universities) that I was an impoverished writer and couldn't afford to pay much (it helped that the book was published by the US National Academy of Sciences press, the Joseph Henry Press). Even so, it was quite a strain on the finances.
Luckily I only needed one photographic illustration for Before the Big Bang and that was courtesy of NASA who, bless 'em, don't charge if you credit them appropriately. But I was worried for a moment there.