I changed the 3 core files:

/lib/filemanager/ImageManager/image.php
/lib/filemanager/ImageManager/assets/imagelist.css
/lib/filemanager/ImageManager/assets/manager.css

The original Image Manager uses a 5 column table to lay out the thumbnails. 
This left alot of room on the right hand side with larger displays and my
clients are always asking me why the images tile incorrectly, not realising that
is as intended.

I switch from using tables to using inline-block divs.  It now expands all the
way to the right hand side and wraps around.  The divs have a fixed height
although the width is flexible to support long titles.  The titles do not wrap
to help preverse vertical spacing.  The thumbnails are centered above the
titles.  I've tested this personally on IE9 and FF12 and I've had it tested
remotely using the latest Safari by somebody else although I haven't seen it
myself.

The changes have been made the the 1.10.3 version.  I hosted the zipped changed
files and screenshots on one of my test domains:

http://www.ninethree.co.uk/uploads/cms-forge/fullscreen-image-manager/full-width.jpg
http://www.ninethree.co.uk/uploads/cms-forge/fullscreen-image-manager/long-names.jpg
http://www.ninethree.co.uk/uploads/cms-forge/fullscreen-image-manager/scrollable.jpg
http://www.ninethree.co.uk/uploads/cms-forge/fullscreen-image-manager/FullWidthImageManager.zip