Add this node in front of a Scale node to get a pixelated (non-smoothed) image from the resultant upscaled image.
Standard image input.
This node has no properties.
Standard image output.
In the Compositor, check the Use Nodes checkbox and add an input Image node and an output Viewer node. Connect the Input node to the Viewer node and check the Backdrop checkbox in the header. Open an image you would like to pixelate using the open button on the Image node. This image should now appear in the backdrop. Now add two Scale nodes between the input and output. Change the values of X and Y to 0.2 in the first scale box and to 5 in the second. The background image will be unchanged.
Now add a Pixelate node between the two Scale nodes.
You can use Alt-V and V to zoom the backdrop in and out respectively.