I think the background ended up being white because you put it into MS Paint and it lost its transparency.
I thought the fact that there was no colour around the numbers and graduations would indicate that they were transparent, so that's why I didn't say so. Maybe I should've done this...
Transparent ruler
I can well believe that you have thousands of files and folders on your machine, I think you have posted most of them on here! Only joking, maybe 95%, hehe. : ))
I'm surprised that you have Photoshop, that's expensive, make the most of it.
PNG is one of the filetypes that support transparency if you wish to add an alpha (transparent) channel to your picture.
Not transparent jpg
Transparent png