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.
Before the change, you could use the operator filetype: in Images looking for image types such as PNG, JPEG, BMP, ICO, GIF, etc. – you still can; for example, bank “new york” filetype:ICO.
Now, Google will respond to TWO filetype operators to search for images of a given type in documents of a given type: filetype:pdf filetype:png butterflies.
I sense that it is a work in progress; Google may be re-implementing its “file types” found in search. (We have seen some early signs before). This search for two different file types in the “main” Google now brings results: filetype:pdf filetype:html healthcare providers. (Expect another post. 😉 )
It is interesting and worth some research time!