with still images the criterion important are related to image/file load time.
as the still image file is loading all other playback has to wait-
still images take a lot longer to load than movie files.
Usually load time is related to file size but not always
jpeg files are the smallest in size with compression between 50-70%
tiff and photoshop files can have a lot of additional stuff inside them- photoshop files contain all the composition layers in addition to one composited image.
PNG files can be quite good - and compression algorithms to make small files for web use have got very good.
PNG will also do transparency - and you can create a PNG movie with transparency- see image below -
you will usually get much better performance overall in catalyst if you turn any still images into movie files.
and even better performance if you have a series of still images you need to step through- if you turn all the images into a single movie - then step through the frames using inframe
load times for movies frames are much shorter than stills
even at large image dimensions say 4096x2048 - a movie file will load and display faster.
So turn stills into movies for best performance