Q:

How can I represent speed in a graphic?

How can I show speed in this example?
I don't have any vector programs, I'm working in Photoshop.

Any advice will be helpful. 

A:

Create a teardrop shape with the pen tool:

Fill it with white (Command+Delete).
Duplicate, stretch and rotate your copies to match up with some of the lines in the object:

Merge the shapes together by selecting all of them and 1.hit Command+E or 2. right click and choose Merge to Smart Object.
Then add a layer mask and put a slight gradient from the bottom up...

A:

An alternate approach is to provide an external point of reference.  
In this case, the viewer is traveling at the speed of your subject. Stationary objects appear to move in the opposite direction as indicated by the clouds below.  
You'll see this approach in Anime, where the example provided by @Dalvenjia is possibly more commonly seen in cartoons from the west. Not to imply that this is a better approach, but certainly takes on a different stylistic character.

From a technical point of view it's essentially the same.  
I created a few layers with clouds using the brush tool, then apply a motion blur to copies of each layer.  Shift the alignment of your motion so that it fades away in the direction your subject is moving.  
Hope this helps.

A:

As Scott commented, motion is usually illustrated using lines, blur and/or translucent partial images of the object trailing behind the "direction of motion."
To end up with a clean leading edge with a motion-blur or similar effect on the trailing edge, you need two layers. The first is the object itself, the second is laid on top and partially masked out.
One quick and simple way to do this is with Motion Blur. Here's an illustration, copied to a second layer with a motion blur applied to the copy.

And here it is with the "leading edge" blur removed with a layer mask:

By feathering the mask, you can adjust how much or how little of the effect overlays the object. For extra credit, you could apply a Liquify filter to the tail end of the blur, to create "turbulence."

