Interesting idea if possible, although I'm not sure how efficient it is going to be to load a huge .png file repeatedly in order to crop out a little chunks of it to display. This is not a game engine where you can load a "bitmap" one time when the app starts, and then just use that single memory copy to "cut out" pieces to use. Every meter using that image is going to load the entire image.
I loaded the same 5meg .png file three times in three meters, and as I added each one the amount of memory Rainmeter was using increased by a bit over 5meg. ImageCrop'ing one or more of the images made no significant difference. In fact it made it grow more, as I suspect that in fact it now had TWO copies of the cropped image in memory. The original, and the cropped version.