I assume this rnd goes from 0 to 0.99?
so extremes are int(0 * 2) = 0 + 1 = 1
and int(0.99 * 2) = 1 + 1 = 2
I think you would be faster just doing
rnd and do
if stDisplayImage1 < 0.5 adsense 0 else adsense1
then you could also assign probabilities of 0.6 0.7 etc if one of them proves more succesful IF sometimes interchanged with the other just to have not always the same on the spot to draw attention.
You might want to check out also other distributions like beta but the differences might be academic.
<%if stDisplayImage1 = 1 then%>
----adsense code #1
----adsense code #2