I have an easiest way to do this:

LOAD( url=URL(a='app', c='controller', f='foo', user_signature=True), ajax=True, content= IMG(_src=URL(request.application,'static','images/loader.gif'))

Explanation:

The full signature of the LOAD helper is the following:

LOAD(c=None, f='index', args=[], vars={},
     extension=None, target=None,
     ajax=False, ajax_trap=False,
     url=None,user_signature=False,
     timeout=None, times=1,
     content='loading...',**attr):