sectionimage question

[dmgd]

First THANKS for a CMS that I can explain to clients and they understand and can maintain thier own site.
Problem:  I try to modify sectionimage function so I can use in a style  url({sectionimage}) tag.  All I changed was the echo ""; line to echo "url( . . .) top ";  When I upload the function with a different name function.divsectionimage.php and go to admin|extention|tags I get a blank screen.  I can delete the uploaded file and everything is back to normal. 

I tried creating a user defined tag using onlly the function smarty_cms_function_sectionimage($params, &$smarty) part of the tag but this produces nothing.

I am clearly missing something about creating plugins here.  I have searched and can not find an article about creating tags, just how to use them.  Would someone point me to some help on this problem I am haveing.

mark