Now, you may all know this already but I discovered by accident (after months of doing it the hard way) that if you just highlight the title of the post/recipe/book review and then push the Pin it button on your toolbar the words you highlighted will turn up in the text box of your Pin – you don't even have to copy and paste them.
Clever, clever Pinterest just knows that's the description you want with your Pin. If you don't highlight anything Pinterest will grab the title (whether visible or not) of the photo.
So if you want to make pinning really easy for people, you need to label each photo with something meaningful – ie not IMGxxxx etc. You don't have to do this by putting on a caption – I just click the pic and then Properties . Then there are 2 boxes – I put the title of the post into the one called Alt Text and then click ok. This doesn't just help with Pinning – it also improves your overall SEO (Search Engine Optimisation)