[ferriswil]

The "free" in this case is a verb. The objects are freed from the background.

[Retr0id]

Until your comment I didn't realise I'd also read it wrong (despite getting the gist of it). Attempted rephrase of the original sentence:

Imagine history documentaries where they take an old photo, free objects from the background, and then move them round to give the illusion of parallax.

[necovek]

I'd suggest a different verb like "detach" or "unlink".

[tzot]

> Imagine history documentaries where they take an old photo, free objects from the background

Even using commas, if you leave the ambiguous “free” I suggest you prefix “objects” with “the” or “any”.

[nashashmi]

Free objects in the background.