This is an updated version of Blamking's program, Woolify. The program created pixel art for any image provided by the user to create in a world (the program doesn't put it in the world, you have to do it yourself, by hand). He hasn't updated it in years, so I decided to do it myself. A few things to note:
1) 99% of the code was written by Blamking; I only added new blocks and updated the hues of existing ones.
2) The blocks chosen are ones that I think are the easiest to obtain in survival. To find replacement blocks, click here.
4) There is no guarantee that I will update this again.
Blamking does a great job of explaining the functions of this program, so I'll quote him from the original forum post, which can be found here:
Load just about any image by clicking the "Load" button. While not necessary, it may be helpful to crop your image in an image editor before loading.
You can choose most of the blocks available from the pane at the right. Either scroll while your cursor is in this area or click the arrows to reveal more blocks. Next to each block checkbox, it will show how many of each type is required. You can also change the values in the "Weight" fields to change the number of occurrences of that block in the results. For instance, entering 2 for the white wool weight will cause the results to include much more white wool. If you are playing creative mode or are otherwise not restricted, the default values should be fine in most cases.
You can change the size of the result by entering different values under "Dimension". Enter 0 here if you want to match the dimensions of the input sprite.
Check "Original" to toggle between viewing the input image and the output image.
Check "Dither" to toggle between dithering and non-dithering. I find that dithering is best for photographs, while non-dithering is best for sprites.
If you are building horizontal pixel art, and want to take advantage of shading caused by blocks placed above the art, check "Shade". Checking "Show Shade" will display a black and white image indicating where blocks need to be placed above the art.
You may press the "Save" button to save the currently displayed image. Note that this only saves an image - this program cannot export to MCEdit files.
You can move the image by left-clicking and dragging. Zoom by scrolling the mouse wheel while the cursor is over the image.
Select a single pixel by left-clicking. You can move the selection by using the arrows on your keyboard.
Select a portion of the image by right-clicking and dragging. This updates the number of colors requires in the pane on the left as well. Move this selection by pressing shift and the arrows on the keyboard. This is useful if you want to build chunk-by-chunk. To reselect the entire image, right-click once.
Pressing 'G' on your keyboard toggles a grid on the image. 'N' toggles between color names and ID's in the left pane. 'T' toggles the pointer tooltip, which shows the color and location of the pixel over which your cursor is hovering.