To capture just the image, you can either download the image and upload it to Zotero as an attached file, or you can insert a copy of the image into a Notes field (see below). [1/4/18 update: No, you can't insert an image in a Zotero note, or at least you can't if you are going to try to sync it to Zotero.org.]
1/4/18 update: According to Zotero, "Notes with embedded images cannot currently be synced to zotero.org."
How to insert an image in a Zotero Note (works with Firefox and Chrome; I have not tested it in Safari)
Pinterest is Open URL-compliant and has a browser translater/icon that will download the Pinterest item you are looking at as a webpage; alternatively you could save it as a webpage. Both methods capture different elements, and neither is comprehensive. I think I'd give the preference to saving it as a webpage, but you will need to carefully add in bibliographic details - which adds to the complexity (author field options are author, contributor, translator -- collector is noticably missing!)... [more later - I need to fiddle with Pinterest a bit more - if you have ideas, please comment!]