Transparency is always a problem, as only a few image codecs support it.

The Animation codec for movies, TIFF files, photoshop files for still images.
There may be others.

The problem with transparency is that it is done with an uncompressed alpha channel.

So this kills performance.

I have used the animation codec on real shows with graphical image movie files - with some success - but animation codec files can give quite poor performance.

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Jitter, you really really have to convert your stills to something more manageable.

Still image files, tend to be much larger than movie frames - and take a long time to load.
the hard disc in a computer can only do one thing at a time....