I know that to output all images (.png) in a single folder is
but I can't figure out how to check my whole hardrive (all folders and subfolders) for .png files.
You need to use the
/s (include all subfolders) option.
You might also want to consider using
/b (Bare format (no heading, file sizes or summary)) as well.
To search the whole of drive C:
cd /d c:\ dir /s /b *.png
To search the whole of drive C, and save the output:
cd /d c:\ dir /s /b *.png > files.txt