"Crusaders of old had a trusty steed. How much horsepower does a Dark Knight need?"
This might be the easiest riddle in the game - just scan the Batmobile.
"This cold corporation changed his life forever, curing his wife now a chronic endeavor."
To solve this one, you need to scan the Goth Corp sign. The building is located in Otisburg; head there and find a vantage point on the nearby tracks to get a good view.
"He lives and dies in seven days, this beast you've tamed but his song still plays."
Check the underground area near Perdition Bridge to find this sheet of metal you can grapple over. On the other side is a door leading to a very small room containing a poster of Solomon Grundy and a record player playing a creepy tune. Scan this setup to solve the riddle.
"Roll up! Roll up! For the circus of strange, this porcine professor is clearly deranged."
You'll have to reach the end of The Perfect Crime side quest for this one, as it will give you access to Pretty Dolls Parlor. When you do, head all the way to back room and look for a circus poster on the wall. Scan this to solve the riddle.
"A psycho killer's grim design, what has an angle but just one line?"
Head to the docks at the northern edge of the island. Move west around the perimeter to find some bodies posed as if fishing. Scan them to solve.