I ended up uploading it, but is it good enough quality for Danbooru?

I don't see why not. I think it looks pretty good.

I think the matter is that there is a larger version available, but he doesn't meet the requirements to get it from their site (ie needs to be more active to get enough points to download it or has to donate for points on their site).

I did sign up an account trying to access a better version, but as Renim suggested, I didn't have enough "points" to download it. Dunno whether it would be better quality if I'd gotten it like that. Oh well.

Here's a megaupload link to the artbook this is supposed to be from. You could try getting the full version from there.

Thanks! It seems all the Agarest-images I found on animepaper.net is within this artbook. Will upload them to Danbooru unless any problems occur.