Hum, she's cute.. but yeah, I can't really see the bondage due to the angle of these pictures. However I agree with what was previously said: it doesn't seem like she's tied to anything (Well, there's that thing around.. I can't tell where it is but it's pretty high and I doubt effective).
Cheap suggestions: Hog ties. They're fairly restrictive with little work.
You might try switching the direction of the chair so that her ankles are closer to the hole, allowing you to apply more restrictive measures. That might be more on the painful side for anyone who fights or tries to pull away, though, if the chair is light enough for her to move.
I don't have much information on where you're filming but you could always try a spread eagle on a bed, or have her lying on her back on a couch. That would allow you to run (rope or another bondage device) underneath the (bed or couch), so that her ankles are tied tightly to her wrists or the posts of the bed/couch/whatever.
You might also consider binding her ankles to the legs of the chair (if applicable; I can't see the chair well enough in the images) while setting the chair's back onto the floor.
Mostly I think you're not tying the right places in the right ways (ankles/knees together, or individually but fixed to something).
Maybe that was somewhat helpful?
Best~
