You can also find unfeatured image URLs by replacing one letter.
For example: imgflip.com/gif/9c1swd
I replaced 'd' with 'c'.
I got this unfeatured image. imgflip.com/i/9c1swc
I find this to be less plausible than my other theory.

Did you send the image like this?
9c1swd.gif (click to show)
Because you can take the "9c1swd" part and reformulate like this:
imgflip.com/gif/9c1swd
You can then upvote my GIF.