I am running the latest version of FrameMarket/Gridmarket and setting up same on a MU installation with Marketpress.
No other plugins are running at this time.
In setup for the child theme I am attempting to set a background image and/or change the background color.
I had previously set the color to: #dfebe6
I have removed that color with the background upload tool and the preview (after save) shows a white background. The front end background color has not changed.
I then attempted to upload a 1440px x 900px test background image which you will see in my screenshot below. This image seems to have been reduced upon upload as you can see.
The site is: http://marine/ac.com/marketplace
Wordpress is the latest version, it is a brand new clean install.