If you’ve already created an Instagram account, take 300 seconds and reply to me with your account name. Or follow books_and_whatnot on Instagram, and I’ll follow you back.
That took about 45 seconds. With the reminder of the time, look at your website, Facebook page, newsletter template, in-store signage, and see where you want to promote the fact that you have an Instagram account.
Instagram provides these and other branding images which can be downloaded from their website here.
Screenshots. (Or even better, create your own screenshots with your identifiable bookstore content.)