Ah, ninacoon, thanks for the vote. You seem to have one of those cumulative pieces of writing where information about the subject comes in bits in the terms successively applied, as well as explicit description/definition - so that the Grabdenkmal (1, & I stand to b corrected, any tomb statue/sculpture or 2) the tomb as a monument in itself) may in this case be a Grabkapelle, our funerary chapel in the church, and, again in the case at hand at least, over the Gruft. Or the chapel could be in the Gruft (Gruft as crypt). Yet again this is all without specific research on my part, so "ohne Gewähr & bitte konsultieren Sie Ihren Arzt/Apotheker".
Your Gedächtnisbild **sounds** like yer everyday commemorative/memorial portrait, be it a painting or (less likely) a relief - with a view behind the portrait, of the church interior (probably he was a donor). Sorry to be so lazy re. sources & verification, but at least that's where the language takes me. I'd presume Erbbegräbnis to be synon. with Erbgrab/-gruft, hereditary tomb, crypt or whatever. Wikip. confirms:
http://de.wikipedia.org/wiki/Erbbegräbnis (googling any of those Erb-* terms brings up a full yield)
Best / S.