How to add a button for a mod?

Ok, I have decided to turn a bit of code i use on all sites i make into a mod.

The only problem I am having is, how do i add the additional button to zencart.

is there a way to add the button definitions, as i don't see an obvious override

Normally i just use a simple image link as all the stores i have developed for are english only

basically like
<img src= "blablabl/buttons/mybutton" />

whereas if i want to be incluseive of all languages I should use something like this

zen_image_button(BUTTON_IMAGE_BLABLA, BUTTON_BLABLA_ALT)

I know it is probably patently obvious to others, but i can't figure where to define the bits, i don't want to override core files if possible

Note the mod is a catalog side mod, not admin.

If it's language-specific, as buttons usually are, then adding a custom file to the extra_definitions folder is the simplest way to do it.

Thanks for that Dr