I use models of a trees with leaves, which each leaf is a picture of a leaf. In the leaf image background should be transparent, but it looks black, as shown in the attached image.
increase your 'ray bounces' in lighting settings.
If that doesn't work then you need to make sure that you're using an opacity map. Change the material type to 'Advanced' and insert an opacity map. If your leaf texture has an alpha channel, then just check 'use alpha from diffuse' in the material properties.
Thanks Willchocks7, it really helped.