Is it possible to use a small picture instead of a > or other symbol on the command bar?

I assume it is done in the cursor for commands box under the interface tab?

Thanks in advance!

Yes.

<span><image src="https://secure.gravatar.com/avatar/4b45a3afd18077f76f6569225c422bdb?d=retro"/></span>

Then, if you want it to look like the normal command box besides the image being there:

Add this to a turn script and User Interface Initialisation:

JS.eval ("$('#txtCommand').css({'outline':'1px solid orange','padding':'4px'}).attr('placeholder','Type here...')")

Egg-cellent! Thanks.