However, it seems that in the past few years the results are getting less and less reliable. Images now show checkerboard backgrounds to indicate they are "transparent" but in actuality, they are not transparent. They have an opaque checkerboard background.
Is there an image search service that shows transparent image results, testing the transparency is true before presenting results?
Beyond that, it's up to you to actually go to the indicated site, find the image and see if it's publicly available.