Hi,

you'd need to implement IPostBackEventHandler interface, plus generate
script calls into onclick attributes of images and divs (is it div
containing two images? should it be distinct which one was clicked or just
the div is enough?)

You can make the script call with Page.GetPostBackEventReference (in ASP.NET
v2.0 it's Page.ClientScript.GetPostBackEventReference) and if you need to
distinguish between clicked elements, give that method an argument
indicvating which one was clicked.

When you implement IPostBackEventHandler interface, given argument is passed
there (e.g IPostBackEventHandler.RaisePostBackEvent is raised when the
element is clicked and postback script run) so you can raise your own event
from there.

Tell a bit more about the images and div, and I can give you a code sample.
Here's also a good  stuff to look at

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cp
conReceivingPostbackEventNotifications.asp

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cp
congeneratingclient-sidejavascriptforpostback.asp