Dutton Peabody: Liberty Valance defeated. D-E-F-E-E... T-E-D? The unsteady hand betrays. What's the matter, Mr. Peabody? Are you afraid? The answer is indub... yes. No courage left. Well, courage can be purchased at yon tavern. But have we credit? That is the question. Have we credit? Well, credit is cheap. Wait for me, old servant of the public weal. Our shining hour is yet to come. As for you, Horace Greeley, go west, old man, and grow young with the country.
Liberty Valance: You lookin' for trouble, Doniphon?
Tom Doniphon: You aim to help me find some?
Ransom Stoddard: I don't want to kill him, I just want to put him in jail.
Tom Doniphon: Ohhh.
Tom Doniphon: Liberty Valance's the toughest man south of the Picketwire - next to me.
Tom Doniphon: Whoa, take 'er easy there, Pilgrim.
Dutton Peabody: Hallie, please! The proprieties concerning the cutlery. Now, how many times have I told you, the fork goes to the left of the plate, and the knife goes.
Hallie: What's the matter with you? You superstitious or something?
Liberty Valance: You got a choice, Dishwasher. Either you get out of town, or tonight you be out on that street alone. You be there, and don't make us come and get you.
Tom Doniphon: I know those law books mean a lot to you, but not out here. Out here a man settles his own problems.
Tom Doniphon: You talk too much, think too much. Besides, you didn't kill Liberty Valance.
Link Appleyard: The jail's only got one cell, and the lock's broke and I sleep in it.
Ransom Stoddard: You're not going to use the story, Mr. Scott?
Maxwell Scott: No, sir. This is the West, sir. When the legend becomes fact, print the legend.
Answer: At first glance, it looks like the word "LEFT" in large, widely-spaced lettering. It's very unlikely that this would be a blocking cue, telling Jimmy Stewart to move left (which he does from that point). That's not the way blocking cues are done, for one thing, and a seasoned actor such as Jimmy Stewart wouldn't need such a cue. It's also very unlikely that it's a warning message from John Wayne who is hiding in an alley across the street, on Stewart's right. John Wayne didn't want anyone to know that he actually killed Valance, so he wouldn't alert Jimmy Stewart with any messages scrawled in the street. All hypothesis aside, it's probably just footprints in the dirt, an illusion of light and shadow.