Hi prigupta2
Joomla :) , I don't think you gonna find much help here regarding joomla. You best posting your question somewhere on joomla forums
http://forum.joomla.org/ and reading joomla documentation
http://docs.joomla.org/Main_Page
I did some joomla in the past and I know that there is loads galleries done for joomla. In fact, the best approach with it is if you need something its probably been done by someone before so .... google for it. Just look for modules and components for joomla.
Btw if you need to insert a table using that HTML editor and don't see button for it I am sure there is HTML button. When you click on it your editor becomes plain text editor so you can write code for your table.
Hope this helps...
Good luck