Fixing broken PNGs

Link to Issue: #216 

A broken PNG was reported on master, which led to the discovery that all PNGs in the assets directory were broken on rails-3.2. This pull request fixes those broken PNGs by copying good PNGs from the earlier rails2 branch. 

The issue was most likely caused by the LF vs. CRLF line endings issue, which caused the PNG binary files to be edited and consequently corrupted. A line in the .gitattributes file marking PNG files as binary should prevent this issue from occurring again.