The logic works like this:
{{#ifeq:{{{1|yes}}}|{{{1|no}}}| first param was defined (and may be empty) | first param was not defined }}
Call this template with no parameters and you should get a green check mark. Or, explicitly supply the parameter as 'yes'. Supplying the parameter as 'no' will produce a red X. Other paramters are undefined unless there is a Gtk-parameter.svg image in this wiki
Examples
{{checkmark}} or {{checkmark|yes}}
{{checkmark|no}}