Restored the missing image 'tipsy.gif'

This is to solve issue <a href="https://openhatch.org/bugs/issue842">#842</a>.
The missing image tipsy.gif was mistakenly deleted by the commit 12f5c03.  The missing image causes a 404 when hovering over some places on a project web page (for example, hovering over the page /projects/GeoTrellis).  tipsy.gif was found after checking out the commit e50d226, and by restoring that image now the request for tipsy.gif is becoming a 200 instead of 404.