^
I have a Yamaha RX-A820 but I suppose your RX-A1000 could be rather similar..
To be ironical, you should really be lucky that the Yamaha displays meta data or album art at all. In my experience a lot of receivers are really bad at implementing this part of the specification. So getting something at all is quite a cause for celebration
However, putting aside the irony, the problem is as follows: The UPnP specification says that the server may "offer" album art in various sizes/resolutions (basically "small", "medium", "large"), and it says that the renderer may choose whichever of those offered image sizes it wants to download.
Now MC is a very good server implementation and so it does indeed offer several image sizes (at least "small" and "large"). But the Yamaha (and many other renderers) are really bad renderer implementations, and instead of checking for multiple offers and choosing the most suitable size, it downloads the first offered image. And since MC offers "small" before "large" the Yamaha therefore downloads the "small" image..
In theory MC could be reprogrammed to offer "large" before "small". And in the case of the Yamaha, that might be sufficient to trick it into showing the large image. But the problem is that many renderers either don't expect to see multiple offers at all, and/or expect the first (or only) offer to be a small image. And indeed many renderers cannot scale or display images above a certain size. So if MC would be reprogrammed to offer "large" before "small" it might fix the Yamaha, but would certainly break other renderers.
So this is not something you can blame MC for. You should rather complain to Yamaha (and indeed all other render manufacturers) about this. Most likely they will not patch their existing products, but there is at least a chance that when the time comes for you to buy a new renderer they may have fixed it in their latest models.
Personally I get really irritated about renderer manufacturers because their marketing departments plaster stickers like "DLNA Certified" all over their products, but their technical departments seem not to have read or understood the DLNA Certification Guideline, and certainly have not implemented it in their products.
{ an alternative suggestion: if your PC has an HDMI out, you could hardwire the PC to the Yamaha, and a) get the full MC display, and b) get better general control of the playing experience.. }