Most good quality image files are much bigger than movie files. It takes a palpable amount of time to:
a) Load the image into RAM
b) Decode

The way around this is to convert your stills in to Photo-Jpeg Quicktime movies. This is actually what the Photo-Jpeg codec was designed to do in the first place. The advantage of using Photo-Jpeg over DV, is that you are not restricted to using Standard Definition. Photo-Jpeg will successfully encode movies at whatever size you set. The compression ratio is very impressive for the quality you get. You can then use Inframe to select the relevant still.