SHADOW BALLOON BOY'S ORIGINS
(OLD INFO)

SECTION 2: THE THUMBNAIL

The ICONIC version of the Shadow BB image actually originates from the thumbnail of the video.
This is safe to assume, as the PGB screenshot shows the thumbnail cropped the same way as the available image we have.

The image above is the most accurate image we have to the original thumbnail, which originates from this tweet.
The resolution of this image is 1024 x 576. This is 20% smaller than 1280 x 720, YouTube's thumbnail size, making this likely downloaded from the thumbnail itself.

The thumbnail link has been found posted on the FNAF Wiki here.
In this thread, you can find still-existing images of the original image, however, they've been edited to prove that it's fake.
It's likely that the iconic image was sourced to this thumbnail link most often, instead of the actual video.