For Valentine's Day a couple of years ago, I created this card for my husband, and although I look back at a lot of my digital art and think 'Oh dear!', I still really like this.
This is how I created it...
First I opened the image up in GIMP (free software) I cropped the image and then clicked Layer > New Layer and clicked to create a transparent layer. (You may want to name the layer so you know which you are clicking on later)
To make sure that you are drawing on the transparent layer click Windows > Dockable dialogues > Layers and make sure you are clicked on the box that is a grey chequerboard. You can then pick how big you want your brush to be and trace over the image. I think it looks best when you make it look quite sketchy and imperfect.
Once you have sketched over as much detail as you like then you need to go back onto the Layers box and click on your original image and press delete (the little trash can at the bottom). Then click Layer > New Layer. This time you want to create a white layer. Then right click on the transparent layer and click Merge down.
Although, I do count tracing in art as cheating, I do think this is pretty cool and simple enough for anyone to have a go!