Here are two samples made from illustrations I had laying around. The trillium is a combination of pen and ink, watercolor, and colored pencil. The poppy is done with colored ink only. The inkwork gives both of them a similar strong texture, but the trillium has a somewhat greater color range due to the watercolor and pencil.

I chose these two images because they represent my own favorite styles, and because they bend to digital manipulation quite easily. Both look surprisingly good in 8-bit color. As far as file size, both are saved as 30% quality .jpeg's. Each one is less than 4K, so you could have quite a lot of them on the same page. (This is only relevent for the web end, of couse, you could print the seed packs with the full size, flawless quality, but half a megabyte .tiff files I made the .jpeg's from)

As far as design of the seed packages, I think that you want the picture to be the primary selling point. I'd have the front of the seed pack with the illustration taking up nearly the whole front, and then just the name of the plant in a pretty font below. You can have cultural info on the back. After all, you can buy a pack of plain old poppy seeds with a photograph on the front from a million different places, but with beautiful illustrations you could probably get people to buy the seeds just for the picture.

If you don't like these particular illustration styles, I'm adept at other mediums and could work out something to fit whatever you have in mind.