questions about selfmade icons in the toolbar and menuitems

[Alexander Nagel <feuerschwanz76 web de>]

Hi everybody,
i created a toolbar with custom icons and basically it works quite fine. 
My problem how big should these icons be? In the toolbar i have 24x24 
pixels and in the menuitem i use 18x18 pixels. But for that i have to 
create always two different icon sizes. This is not very good.
Is there there an gtk_... function to minimize the icons?
And what happens when the user uses another desktop theme?
And another question: What picture format/type is recommended?
png or xpm. with or without alphachannel?

Thank you for your thoughts and comments.
greets
Alex