Making a new clip art out of old public domain images is not a hard process. Although on clker.com we are trying to apply more research into automating this process, yet the quality of manually traced images cannot yet be matched.
The example described here is based on this image from the Library of congress and which is shared on flickr:
1. The first step is visit clker.com’s home page and click on Draw.
2. Select the “Image tool” and insert the URL of the image from flickr.com : http://farm3.static.flickr.com/2266/2178246585_14139d6905_o_d.jpg
3. Using thick black lines (size 5), trace all the outside of the balloon using the path tool.
4. Use the node tool (two clicks on the path) while the “link control points” option is set to curve the segments where necessary as you see below:
5. In order to help see the rest of the details, we set the transparency to 50% until we finish the rest of the drawing.
6. Change the color of the balloon to purple/bluish color.
7. Using thin lines (size 2) trace the details inside the balloon using the path tool.
8. Before saving, make sure to return the transparency back to 100% and delete the background image. Make sure to set the bounding box correctly by using the Document Properties -> Canvas Size -> Fit to contents.