CYB-111, CYB-112 Gallery icon corrections.

Images have been converted to PNG and run through ImageOptim.  

arrow.jpg removed entirely as it is not referenced anywhere in the code.

Impacts [CYB-111](https://jira.ets.berkeley.edu/jira/browse/CYB-112) and [CYB-112](https://jira.ets.berkeley.edu/jira/browse/CYB-112)