One feature I missed a long time is to search only for vector images, as eps or ai, and exclude bitmap images.

KB: Specifying e.g. filetype:eps in your search is somewhat effective, although some non-matching images still get through if they've got the same extension in their filename. (This is available as a limited drop down – with only BMP, JPG, GIF and PNG – in the Advanced Image Search page.)

I tried it before, doesn't work in practice. You can't combine several filetypes, like ai, eps, dwg, dxf in one question.
And majority of the people doesn't seem to know the difference between bitmap and vector images. So even if the file has an extension like .ai or .eps, it's almost always a ordinary bitmap jpg-file.

i would like more the oposite search – very often i search for e.g. sketches, logos etc, but for some terms it is almost impossible – gif/png filter returns almost nothing, jpg got first 100 pages full of photos

One approximation to select photos only would be to analyze EXIF tags in JPEGs. Screenshots, logos etc. in most cases won't have EXIF at all, or maybe a default one created by Photoshop etc, which is easy to detect.

Marcin, That would work with original photos but probably wouldn't catch resized photos used on blogs or news sites.

Colin:
Depends on the way they have been resized, but actually lots of software is EXIF-aware and it will not strip the metadata, unless specifically asked to do so. Certainly, many photos would be skipped anyway.

On a related note, searching by EXIF tags would be a cool feature Google might want to add. For instance one could select images from a certain camera model or those with embedded geographic coordinates.