using image control at run time

i have a image control
based on the condition  on server side i need to add the image url
img1.imageurl="/images/y.gif"
or
img1.imageurl="/images/N.gif"
only one out of this 2 shld be visible
i tried in this way
if condition 1 then
img1.imageurl="/images/y.gif"
else
img1.imageurl="/images/N.gif"
end if
but how do i display it
 
...