You have to check if the mouse is over the image and if the player has clicked. Like this:
Do
mousex()= mousx#
mousey()= mousy#
imagex1#= (the x coodinate on the left side of your image)
imagex2#= (the x coodinate on the right side of your image)
imagey1#= (the y coodinate at the very botom of your image)
imagey2#= (the y coodinate at the very top of your image)
if mousx# > imagex1# and mousx# < imagex2# and mousy# < imagey1# and mousy# > imagey2#
Bttnclick=1
endif
IF Mouseclick()=1 and BttnClick=1 then gosub YourSub
Loop
Hope that helps.