How to share bigger image to Facebook and Twitter

Hi,

How do I change shared image size to facebook and twitter?

At the moment it takes thumbnail and shares it to facebook. It will be small and will not be atractive to facebook users.
 
What i need to do to get a proper gallery for my forum???

Xenforo is great, but the gallery is a real s***t. Sorry for my language, but XFMG is not worth buying.
 
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