Can't get my custom header to override "gridmarket" header image!

Hi, I'm using FrameMarket with the Gridmarket child theme. I cannot get the header to change to my custom header image! I've tried many times in the Theme Options dialogue area, Custom Header uploads, standard Wordpress page headers, etc. and either it just doesn't appear, OR I get an error message that the image is the wrong size and can't be used or cropped (it's very small and would fit in the space where it now says "gridmarket.") Very frustrating -- any suggestions please?
It almost seems like the header image, "gridmarket," is dynamically called from WPMU DEV server and therefore inaccessible, which is very frustrating. Thanks in advance -- I realize it's sunday!