There is a third possibility. A JPEG picture tile consists of a linear combination of 64 standard tiles. If you only compute a few of them you could display an approximation of the picture somewhat faster.

This is (if I understand it correctly) similar to the method in progressive encoding only that with this method you have to read in the whole file. With progressive encoding the coefficients of the standard tiles are grouped together so that you only have to read part of the file to display a first approximation of the picture.

That presumes that displaying the pictures is not completely IO-bound. Just tested it on my machine, konqueror showing only thumbnails of hundreds of pictures got to 72% CPU usage, so I think showing the full pictures could be stressing out the CPU.