Firstly did you try the Shaders in the Browser ? 

Well, otherwise you can easily make your own custom leaves in Carrara to work with textures.  Basic instructions are given in the manual - try page 96.  In older versions of Carrara you saved a scene with a leaf as the first and only scene object to a particular folder, with Carrara 5 you can import the leaf from a file in the tree modeler window.

As far as I know all of the standard leaves are polygon based and don't use texture co-ordinates so you can't just stick a photo on them.  However the trunk and branches use a standard texture space such that the image is wrapped around the branch once, like a sticker on a can.  The bottom of the image is at the base of the trunk or branch node and the top of the image will be the tip of the branch. 

So for bark textures what I do is paint or take a photo of the tree trunk, a thick branch and a thin branch.  Make these seamless in a 2D paint application.  Apply these as tiled texture maps to the color channel of trunk, branch1,branch2 respectively and set the tile repeat parameters to get a well proportioned result.  In other words you can only take a photo of one side of a tree and perhaps half a metre high so wrap it round twice and repeat vertically 10 times for a tall straight trunk.

To create a custom leaf to which you can apply a photograph, start a new empty scene.  Insert or drag and drop a Plane primitive into that scene.  Save the scene to the Carrara 5DataPlantsLeaves folder with a name like "UVleaf" (make sure you tell it to save with a scene preview).  It will now come up in the list when you hit the [Choose a leaf] button.  In the Shader room, if you started the tree with one of the imported shaders then you will see the list of parametric mappings applied to the various tree parts.  When you apply the leaf as a texture map then the middle left side of the image is where the leaf stalk must be and the midrib runs horizontally across the image.  Use the rotate texture map option if your leaf photo is facing the wrong way.

Use an alpha channel to make the leaf margin.  You will need to change the render settings for best results with transparency on and increase the maximum ray depth to the highest setting for a deep forest scene.

Another method would be to make the whole leaf in the vertex modeler, import a trimmed down leaf photograph as the backdrop then use the draw polyline tool to trace all the way around it.  Use the planar projection method in the uv editor to get the texture co-ordinates.

You can get a variety of coloured leaves with a few tricks, try painting or tinting two versions of the layout, one green, one red/yellow and then use a mixer driven by a blobbly 3D fractal shader.

I hope that gives you enough ideas to work exactly how to do it all yourself  !