This instructable describes how to use a random googled image as a face for a Jack o' Lantern which can be 3d printed!
Download an image with a reasonable high resolution. Open this in the great free image editor Irfanview. Crop the image and convert to grayscale.
play with brightness and contrast untill you have a nice black and white image.
Copy the image into a vector editor like the free "Inkscape" and trace the bitmap to create a vector image.
Make sure the vector result consists of only single lines! Simply delete lines you dont want. Save the vector as DXF.
Add an extrude to the front-plane of a pumpkin model in your favorite CAD program. (I use CREO which is difficult to master but is fast and flexible)
In the extrude sketcher, import the dxf. Be careful with importing large files, as every line has to be dimensioned by the CAD program, this might take a long time or even end the session..
When successful make sure all loops are closed.
Edit the sketch a bit to optimise the extrude untill you're happy with the end-result. Export to stl to 3d-print the model!