The Pixel Art design tab has been created to easily allow you to create 2D pixel art, or 'bitmap' images in Minecraft. This tab is used as follows:
Click the 'Image' display in the Configuration panel to choose an image to base your pixel art off of. When you click 'Open' the image will be displayed in the 'Image' box, and you will be told the width and height of the selected image.
Once the image is loaded you can choose the size of your block grid. By default 'Lock aspect ratio' is checked, so entering either the width or the height will fill the other box for you based off of the aspect ratio of the chosen image. You can turn the aspect ratio lock off if you wish, but you'll end up with strangely distorted images. Once you have your grid width and height selected, click the 'Generate' button.
You can now start placing blocks on your design grid in whichever way you feel is most pleasing.
Don't forget to occasionally reduce the opacity of the image you're working off of so that you can better check your work. The Opacity slider at the bottom of the window allows you to set the background image opacity to whatever level best suits you.
Once done you can save your work as the Minecraft Structure Planner standard XML format (if you wish to further tweak the design in the 'Freeform' tab), or export it to the NBT based schematic format if you wish to import directly to your game world using MCEdit.
Now all you need to do is either use the plans to build your pixel art in your world (the 'Material Requirements' panel at the bottom of the screen will show you how much of each type of material you'll need to build it), or import your design directly into your world using MCEdit.