That indeed seems like a transparency issue. To be exact I think the transparency image needs to be inverted (black leaf and white edges from white leaf and black edges).
Have you been able to isolate which texture is causing the problems?
Usually there's a shader for stem, petal, leaf etc. So which one has the error?
If you have isolated it then open the shader and first you'll see the diffuse colour texture.
If it is a .tif file (XFrog models usually come with .tif texture files which have an alpha channel for transparency) then you probably should see the same texture appear in the opacity tab. If so, then check "use transparency from alpha" (or something along that line, doing this by heart now).
If it is a .jpg/bmp or .tif without alpha channel then open the texture in an image editor like Photoshop and prepare the transparency image yourself.
You can do this by making the leaf completely white and the surroundings black. Select the white area and shrink it by 1 pixel or 2 extra to make sure that no white goes beyond the leaf's edge.
Another option is to open the .tif from XFrog in an image editor and save the alpha channel as a separate .bmp/jpg and load that separately into the opacity channel of the shader.