Are you using DLNA? If I do, I get only low res cover art pushed and shown on my renders (Squeezeboxes and associated apps).
The DLNA specifications allow Control Point and Digital Media Servers to offer album art in various pixel sizes small, medium, large (i.e. resolutions).
And last time that I looked at it I think MC was indeed offering all those sizes, and is indeed capable of serving all those sizes.
The problem is with the renderers (as usual). Many renderers cannot handle medium and large size images. And many renderers will only (try to) display the first image offered (whatever size that may be). So MC's strategy to try to support as many renderers as possible is to always offer the smallest image first.
{ Apropos Whitebear: this parses all the image sizes offered by the Control Point or Digital Media Server, and it selects the largest size out of what is offered regardless of the order in which they are offered. So if you see only a small size / low res image, then it is possible that MC may have changed its behaviour since I last looked into the topic. Perhaps one of the JRiver guys can comment on this? }