Using SS3 I have added 37 image files inserted during editing pages. The first 15 can be seen in [site]/admin/assets/show/1, but when clicking next page it is completely empty. I can see the files in ftp, they all have the same owner and permissions.
A workaround is to create a folder and move some files into it, then some of the "invisible" files appear at the bottom of page 1!
The site works fine, all photos show fine.
EDIT:
It seems like the thumbnail creator is very memory greedy. I hope it will be solved by this issue [http://github.com/silverstripe/silverstripe-framework/pull/2859] in version 3.2 (now in beta1 2015-06-21). You can workaround by sorting files into folders and keeping the file size low.