Hi

I would do this using triggers - only available if you have PowerPoint
2002/XP or above.

Insert your big and small pictures using Insert->picture->from file
Right click on the big one and select custom animation
Add effect->Entrance->appear
On the drop down list next to the animation select timing->triggers->start
on click of->select small picture from drop down list
Select big picture again
Add effect->Exit->disappear
On the drop down list next to the animation select timing->triggers->start
on click of->select small picture from drop down list
Repeat!

Working with multiple objects can get confusing - read this for a great tip
on 'select multiple objects' button:
http://www.rdpslides.com/pptfaq/FAQ00504.htm
or use tab to move through them.

Post back if you have any problems

Lucy