First off, I created a new document and placed the 2 sources on separate layers after having extracted them from their original images. I used layer masks to select around each image and cut them out but your favorite extraction method will work just as well.

Then, I made a selection around the frog's head and copied it to another layer as well as duplicating the raspberry layer and used this duplicated raspberry for the shaping of the head. I placed the raspberry over the frog head and lowered the opacity of the raspberry, around 60% or so, so I can see the frog head behind it.

Now I use the liquify too to shape the raspberry around the head. I used varying sizes and pressures so you can experiment with what works best for the given area. I also had the "show backdrop" checkmarked so I could see the froghead underneath it. You can vary the opacities of the backdrop. Although I kept mine between 40-50%. After liquifying, I ended up with this:

Even though the shape is almost right, I need to erase parts of the raspberry to get it to better match the frog head shape. So I ended up with this:

Now that the shape is correct, I need to cut holes for the eyes and the nostril. I made the raspberry layer into a layer mask and masked out the eyes and the nostril. I also need to add some shading and lightening to get some of the features onto the raspberry like that of the frog. With some burning and dodging, alternating between hilight and midtone, and using the frog source as a guide to where the darks are, I got this: