From the theme description which is linked to on the footer of your blog:
Simply visit the theme options or theme customizer page and select your desired thumbnail orientation. http://theme.wordpress.com/themes/gridspace/
That being said, when it comes to setting featured images :
Horizontal Featured Images: 384 x 285
Vertical Featured Images: 248 x 330
Square Featured Images: 384 x 384
Logo: 180 pixels wide, flexible height
Screen options http://en.support.wordpress.com/screen-options/ and modules note that the module you must include is the Pages module.
Page – Likes and Shares, Page Attributes, Featured Image, Discussion, Comments**, Page Revisions, Slug, Author, Revisions, Writing Helper
We Volunteers answering support forum questions don't have access to premium themes. Read-access to the premium themes forum is now open to all users so you can search to see if your question has been previously asked and answered there. The premium theme forums are at http://premium-themes.forums.wordpress.com/ with separate sections for each theme. http://premium-themes.forums.wordpress.com/forum/gridspace
If you are not successful when searching there is a link for the specific Premium Theme support forum for your theme under Appearance -> Themes in your blog’s Dashboard, but do note that the premium theme must be active and you must be signed in as the user who purchased it in order to see it and post to it.