All you need is:
* 2 source images,
* a few layers,
* 1 Clone Tool,
* 1 Brush,
* 1 Layer Blending Mode
* and optionally 1 Layer Mask.
Please forgive my poor English, I hope that descriptions will be understandable.
OK. Open your sources in PS. I used an already retouched photo of the face and a slightly different source for the tongue. I hope you don’t mind :)

Cut out the frog and drag it into the opened window with the face layer.
Rotate and resize the frog.

Add a layer mask to the frog layer and mask the unneeded parts. When you are happy with the result click Menu/Layer/Remove Layer Mask/Apply.
You can just use the Eraser Tool, but masking gives more control of the process.

Here’s a nice tutorial on Layer Masks

Select the tongue from face layer, copy it on the new layer and put it between the face and frog.
Lower the opacity of the [i]frog[/i] layer to about 40%, just to keep it visible, and then transform the tongue layer to fit the frog.

When you hide the frog layer it should look like this:

Now Ctrl+click on the frog layer to make selection and with clone tool fill the selected areas on tongue layer.

Then hit Ctrl+Shift+I (Select Inverse) and Delete. Deselect (Ctrl+D).
You can delete the frog layer too if you want, you won’t need it anymore.
Resize and rotate the tongue if necessary (I had to make it slightly bigger) and blend the lower edge with face layer using layer mask, eraser or clone tool.

Now the shadows. Create a new layer below the tongue (you can name it shadow 1) and paint the black shadow around the tongue with a small brush.
Apply a Gaussian Blur filter (with a rdius of about 2px) and set the layer blending mode to Soft Light.

Make new layer above the tongue layer (name it shadow 2), Ctrl+click on the tongue layer and paint shadows inside the selection (just the edges).
Deselect, apply Gaussian Blur, set blending mode to Soft Light.

Almost done. Now make a new layer above all your layers and call it light. Grab the Clone Tool (just be sure to check the Use All Layers option) and clone some reflections. You can paint them if you want, but cloning is easier.

That's all! The result should look something like this: