Ask HN: How to Search for Transparent Images?

My daughter is trying to find an image of a sunflower seed with a transparent background right now. I showed her the "filetype:png" trick in google image search to get PNG only results.

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?