You don't necessarily need image maps to control translucency, though it might be very useful for close-up shots to get subtle differences in nerves of the leaves etc.
Anyhow, it works as follows:
Default is 0, no translucency
Value of 1 is 100% transmittance of light from the other "side" of the leaf which faces away from the camera.
Obviously this is too much, so for nice translucency I often work in ranges of 0.4-0.8.
If you want to use translucency to brighten up your leafs then you can better first increase the diffuse color of your texture.
The diffuse color is set to 0.5 by default. This means that your loaded .bmp, .tiff or whatever filetype is not being adjusted.
However, if you increase the value from 0.5 to >0.5 you can brighten your texture, without having to do that in photoshop or the like.
Very useful I think.
I hope this makes some sense.
If this doesn't help can you post an image of what and how you are doing it?
Cheers,
Martin