When a software package is declared as open source and sports an open source logo who does it belong to?

like anything else.. it belongs to the Author / copyright holder.

the GNU licenses etc, allow the software to be distributed and copied freely.. providing the source code is provided and all copyright notices are left intact..

you are free to modify the software and distribute your modifications.. but you must provide the original source with the distribution if it carries the same name..

ie you can't modify the script and distribute the modified version and call it XOOPS 2.0.6 or whatever.. but you can if you provide the info to modify the original version (i maybe slightly wrong here) but any modifications/improvements should be made freely available to the public aswell..