We will use a paint package to modify a simple leaf scan, adding transparency, bump, and color variation to our leaf material.
The first thing we need to do is create an alpha mask that will provide the information for Blender to make the edges of the leaf appear transparent in Blender. Unfortunately, there is no single button in any paint package to do this. In the majority of cases, you will have to create a new transparent layer and draw around the edge of the leaf either with a path tool or manually painting the edges. This can be a tedious exercise but will be rewarded later when you use the alpha channel to cut the leaf out from a simple plane. Once you have created that black and white layer, you should refer to your paint package documentation to find out how to convert it to an alpha channel.
In GIMP, it's available from the Layers menu Transparency, Color to Alpha. If you want black to be transparent, choose that in...