Setting up another Style?

Appologies if this is obvious but I'm having trouble creating my own "Text floating:" item for the upload dialogue...so on this page:

I created a class in my css:
.imgupl_style_centre{
margin:0 auto;}
then on page /admin/settings/wysiwyg_imageupload under "Image style:" i entered:

Centre Image with text above/below=imgupl_style_centre

and I got "Validation error, please try again. If this error persists, please contact the site administrator."
I tried:

"Centre Image with text above/below=imgupl_style_centre"

then:

Descriptive Name=imgupl_style_XYC,Other Descriptive name=imgupl_style_ZXC..

and

"Descriptive Name=imgupl_style_XYC,Other Descriptive name=imgupl_style_ZXC.."

still no joy... what am I doing wrong?

ben