Basically you could send the jpg files as they are, you could open them and scale the images to fit the grip icon size. Yhis of course would be very time -consuming and in most cased way too slow.
In case the images are put into the normal image folder (or if the target is 6600 MMC) you would be better off by making the standard thumbnails of the images and to use them in the grid. You could make them and ship them same time as you are shipping the images, but you might need to take account the differences between different phones.
I would basically suggest you to ship the images only, and then make the thimbnails in phone. Takes time but need to be doen only ones for a image.
In case the images are located somewhere other than standard location, you could make the small mbm with same name as jpg and use it as icon in grid, basically there no difference between the thumbnail approach, except that you could put both files in same folder or anyway you want.