Distribution map
With this simple node we will see the "if... else" node
First step create a float value
now you can reproduce this, we will replace the "shape" node by "input" node at the end to have ours custom node
now enter in the pixel processe
first create a get float 2 : "$pos" with sample gray to recup the position of pixels, you need to definite "input image 0" because we want use this one like distribution map. Then create a get float to call the float "value" that you have created. 
now we need to call the both images same idea: "$pos" with sample gray and definite the input image
you can see the node If else, if I translate the graph:
If the pixel of Input 0 is greater or equal at the "value" so use pixel of Image 1 else use pixel of Image 2
Simple !
Now you can try and change the "Shape" nodes into "input" nodes
Cheers
