I'm not sure that I understood your problem, anyway I try put down some ideas that I hope will help...
You can use html DIV (I think that for NS u have to use LAYER tag). U put the big JPEG in a DIV, then u can put dinamically another DIV that contains the jpeg of a green 'x' mark putting his position like this:
....
<DIV ><IMG SRC="BIGMAP.JPG" ALT=""></DIV>
<DIV STYLE='position:absolute;
top:25px; left:30px'>
<IMG SRC="GREENCROSS.JPG" ALT="">
</DIV>
....
in this way u will have a green cross over the big jpeg in the position 25px from the top and 30px from the left of the screen. U must before calculate the position of the green cross for all the organizers, and put it somewhere then everytime u ask for that page u see all the booked and put their div with the right coordinates.
Hope that's one solution 😉