It is new – Google Image search responds to the filetype: operator for document types such as PDF, DOCX, or PPTX.
This is cool because Images is a separate database – its results ranking is different, and extra results may surface. And you can preview the images before opening the results.
It is possible to combine the image search in documents with other parameters, such as image color, size, type, etc., available in the Advanced Image Search. For example, here are large blue org charts at banks: filetype:pdf orgchart bank.