Sounds to me like an event  firing thingy.

Image gets downloaded - fine.

Now if the browser is to opena new window with the detail image then it has to
post the co-ords back to the server.
Fine.
But you have to make sure that the server updates the session vars BEFORE
creating the detail image to return to the client.

If your image control are within the same page and so forth this can be caused
by the event firing order. i.e. your image control tells the session vars to
update after the page has already sent the detail image back from it's
previously stored co-ords.

You could test that by initialising the session vars to something weird - like
the bottom right of the image.

Then, the first time you request a detail image (with proper co-ords) you might
find that you get that one from the bottom right.

Mark - I've been doing some work on this aswell, but client side stuff.

How are you deciding what part of the image to detail ? Are you using an image
map ? I'll post  my code to you when it's sorted out, basically it's javascript
that will grab 2 points from an image when you drag a diagonal across an image
... but there's a small hiccup at the moment - it works - but not well enough.

I have posted a question called javascript - an easy one - have read and see if
you know.

Also - there was no code attatched to this email. If you send it I will do my
best to sort it out. And combine it with my jscript and send it back to you !

Also I don't know why you are using session vars - all you need to do is
get the co-ords and send them in a querystring to have the control immediatley
grab the querystring and send back the image.