The image used in SCP-1000 appears to be a screenshot of the Patterson–Gimlin film, the film that originally depicted Bigfoot.
A brief research suggested that the Patterson–Gimlin film is copyrighted, and therefore not available under Creative Commons. I couldn't find any information regarding licensing in the SCP-1000 discussion page, though I admit I didn't look through every page.
SCP-1000 is tagged with "_image" but not "_cc" which implies that the Technical Team have acknowledged that SCP-1000 contains an image but have not been able to ascertain whether or not it's Creative Commons.
If I were you, I'd avoid using that image, unless someone more knowledgeable comes along and says it's safe to do so.