You need to move them over with the mask in tact (the alpha layer). And you need to make sure they fit the box. To do that, you might need to expand the canvas size of the original, but still keep it square and a power of 2. If you use that right side one, you will have a LOT of wasted space on your others, unless you scale it down, which I wouldn't do. Also remember that they'll be tinted when finished, by the colors set in the SET file for grass color. This means that if you don't use something like 50% gray for your tinting, they'll be either darkened or lightened and tinted with color.
So the image you are cutting from is 512 high. To keep the quality, you'll need a target canvas of 1024x1024 to keep the ratio aspect correct. It might be possible to use a rectangle of 512x1024, and see what it does, but I think your images will be squished. Anyway, the 1024x1024 will have a LOT of wasted space, and you'll see that if you don't pad it to fill the equal width to height, you'll have sparse grass at a high density rating. If you want more density, it makes more planes, which instantly cuts your FPS.
OR
Maybe what you want is to liven up the grass a bit by pre-clumping some grass types in each square of your sprite map. You could put the 512 height one in the middle of the square, and put one smaller thing to the left and right of it. Maybe like another grass clump on one side, and the ironweed on the other side. In another frame, use some other three, or two from here, and one from another. That way when they get put near each other with the density setting, they cut through each other and mix more of the types. Try it out. See what you get and if that's what you really want to do (go HQ), or if you want to stick with lower quality auto-grass and then make a really nice kit of grass placeables.
Personally, I think that image you have is made for two placeables. One made from a two-sided billboard (4 tri) using the tall side, perpendicular to the short one from the top left (4 more tri). Then the right side made into a separate billboard pair (4 tri), with a plane of the white or yellow spray on top, maybe slightly curved (8 tri polygon square with corners bent down). If you want to be really creative, you put two of the spray images on top of each other and offset by a bit so you have flowers that have two colors, where view angle makes them look more realistic.