These are the animation shop settings I use when I save a tag as a GIF.  This usually gives me the best blend for lighter backgrounds in forums etc.  

Choose File - Optimization Wizard.  Then this window should pop up.  Hit "Customize".

This image below is under the "Optimizations" tab.

This is the "Partial Transparency" tab.  If you have a forum or group you're in that has a dark background then you would want to change the "Yes, blend with this color:" option to a darker color.