Here's a nifty trick I found on one of my journeys through Google: if you want higher resolution reference images in your viewport, but want to use the default quality viewport renderer, here's what you do.
- Find the file node for the image and select it. One way to do this is to open up your hypershade, then select the Textures tab, then click on your reference image.
- With it selected, go to Modify/Add Attribute, and put in the following. Long name: resolution. Data Type: Integer. Now hit OK.
- If you did this correctly, you should see your reference image go unrecognizable. To fix this and improve the quality over the default, just change the value of your new resolution attrbute to whatever the width of your reference image is.
Now you should have a reference image that is the same quality as it would be viewing it in an image browser. Credit for this goes to JonasNoell on CGTalk.