I think your'e right Reeba. It is the two ghosts engaging in combat.

Perhaps whilst Manfred's family rule Otranto, they don't own the castle.
Alfonso is 'real owner' so his spirit remains active within.

Manfred is about to pursue Isabella- he declares his intentions above Heaven or hell. This triggers the sigh from Richardo's portrait.

It descends to the floor and marches to the chamber.
Richardo's portrait may have tried to prevent Alfonso from growing bigger. (!!)

The portait may have signed to Manfred to follow it to assist in combat against Alfonso.
Manfred followed but 'the door was clapped-to with violence by an invisible hand'.
Maybe this was the hand of Alfonso already grown.

Yes, I've just found the part Diego sees and Jacquz hears a giant foot and leg in the chamber. (ch.1)

So I guess Richardo lost the combat.
Jacquez says all the portaits were in place so Richardo's portrait is back in the gallery (not smashed to bits !)

I love the idea of the poisoner Richardo dying twice over :-d
but I don't know if it's possible to do so.
(everyone has different notions there).

Yet maybe Alfonso's ghost after fighting it out with Richardo, sent him into perdition, namely to hell. Thus purged Richardo's ghost from his castle !

Do I dare say Richardo had just been hanging round the castle between 'Heaven and hell' !? ;)

Manfred may've set the ball rolling when he declared intentions 'above Heaven or hell.'

Only, how did Richardo's portrait return to the wall ?

Well, I wouldn't put it past Diego and Jacquez to forget they'd seen a picture on the floor and put it back.

Well, proves there's a perfectly natural explaination for everything. LOL ! ;-)