Louisa, if you do not plan on trying to get your book into bookstores or to sell your book at the online bookstores but plan on selling it locally or from your website, then it is not absolutely necessary to get an ISBN for it.

Yes, you can always get one later if you decide that your book is selling well enough that you want to market it to bookstores and online bookstores.

The place to buy ISBN's is from Bowker. The minimum block that you can buy, as far as I know, is 10.

To follow the rules by the letter in assigning ISBN's to books, you have to have a different number for each version of your book: ie -- ebook in each different format needs a different ISBN for each version; the print book needs a different ISBN; etc.

If you took the photograph, you should own the rights to it. However, it might not be a bad idea to check with the amusement park to make sure they have no objections to you placing a photo of their carousel on the cover of your book. I use the CYA philosophy myself. Besides they may want to put your books for sale somewhere in their amusement park if they know the carousel in their park is on the cover.