Usually tiles recalculated when new image comes in, however when 'overlap' option is in effect - tiles recalculated multiple times.
And when layout also has 'zoom' option - when image recalculated it each round zooms itself deeper and deeper, until new images comes and image resets to proper state.
This looks like flicker.

The fix is to always take for zoom calculations real image dimensions instead of previously recalculated.

HOWEVER!
There are too many math and corner cases in mod_conference, so i propose it to be reviewed by widest audience of people who wrote mod_conference!