Aylien lets you analyze and tag images automatically. From a pool of 6000 objects or labels, using state of the art image recognition technology, its image tagging API, automatically assigns tags to images. Candidate labels along with confidence scores are assigned to an image based on what the technology sees. Textual content and images more often than not will appear together on web pages and social posts. Being able to analyze text to understand the context of a webpage is one thing, but being able to analyze both text and images on a webpage can provide a far better understanding of a page.
Diffbot doesn't let you define custom tags. It however does allow you to assign relevant tags to all your images in an automated fashion.Imagga and Clarifai are two products you might be interested in. They both allow you to
Image Tagging APIs