u mean add a ripple to the picture you shown ther? or generally make a water dropping effect? if its just ripples, i believe there is a filter in photoshop that does ripples. if you wanna add a water drop to ur poster, you can layer this water drop picture over your poster, and do some editing such as opacity, fade etc.
I already tried water drop picture, it's nice but I don't wanna take other peoples picture >.< I'm trying to add the water drop effect on my picture, my brother said there's an option in the filter, but I couldn't find it -unsure
haha.. ok.. but thats what most ppl do when they photoshop.. take ppl's pics to create certain effects.. sometimes its ok as long as there is no breech of intellectual property. using the water drop picture is fine in my opinion. but if u insist not to, go to the filter tab, under 'distort', there is 'ripple', assuming u have photoshop cs and above.