Unable to customise Framemarket – Gridmarket child theme


I have installed Frame market and Gridmarket on a Marketpress multisite.

I have followed the instructions for installing Child themes and Network activated Gridmarket only.

My problem is twofold.

Firstly, I have uploaded a new header image to the exact specifications shown 980 px x 100px however, this has been cropped horizontally. By at least 30%. Are the size specs incorrect?

Secondly, despite the upload of a new header, the theme remains unchanged in the front end.

(I have also been having similar trouble with other customising (to footer.php) not showing up as changed).

NB. I do not have any cache plugins installed.

Pleas advise