Problem with Header image Area

I'm having a big problem with header area in the Gridmarket theme. I am trying to change the header on one of the sites within my multisite network and it's not working. After I select the image I want to use, the system is only allowing me to crop 980x100pixels of the image; however, in the custom-header.php file it is set to 980X150 pixels by default! The odd thing is that on the homepage the default header is 980 x 150pixels and it was working fine a few months ago. I'd like all of my users to be able to upload their own headers with the dimension of 980x150 pixels. How can I fix this? Please view screenshot below.

Thanks in advance