Have you tried .png files. They seem to work best.
While I understand you are trying to get the best quality possible, I would focus on the pixel ratio (i.e. 1440×1080, 640×480, etc.) instead of dpi. Again, 1140×1080 is good enough for HD video. Make sure you watch your project on a video monitor.
Try zooming into a video clip. You will notice it looks awful on a computer screen.
Another solution for images at a lower resolution is to duplicate the track when you have finished your edits and transitions. Put the duplicate track above the original. Make the top track progressive scan and set opacity to 50%.

Marc Bowyer
StrobeAlific Media