Finding the image you're looking for can be a little difficult at times, but Google's similar image search is often a big help. If you don't have a similar image, however, the mouthful-of-a-web-app titled "Unofficial Google Image Search by Drawing" will let you quickly sketch what you're looking for and use it as a search query.
The quality of the results you achieve will vary based on the detail and accuracy you're able to achieve with this tool. It can be tough to draw with a mouse or trackpad, and the drawing tools aren't particularly comprehensive, so if you're looking for something complex you might find it frustrating.
Nonetheless, if you're trying to find an icon, a simple shape, a logo, or anything you can easily draw with a simple resizable brush and a few colours you'll get some decent results. While it would be nice to be able to draw a stick figure and get a picture of an actual person, Google's search is pretty accurate and it'll pull the most similar images it can. So long as you understand these limitations, you can use the Unofficial Google Image Search by Drawing to find what you're looking for in a fun and unique way. For a similar tool for programmers, check out previously mentioned Shapecatcher.