There is "Show Object Boundaries" view option inside Blend (under View menu). I didn't know about it until recently. Blend sometimes doesn't show images added to a panel, in this case view object boundaries option is really helpful.
I found this option accidentally when working on carousel control.
Now I have it turned on, and find it very helpful.
See screenshots below.
First one is showing stack panel with several images added when "Show Object Boundaries" option is turned off.
Second screen shows the same stack panel, but this time with "Show Object Boundaries" option turned on.
It is now clear how images are arranged inside the stack panel.
Hope this helps :)