You didn't tell us which iMac you have.
Your screen shot doesn't show any unusually high temps, not even enough to kick up the fans. They usually don't do much until you see temps over 70 C. (I would not give much attention until over 90 C.)
A good tip to remember is to make sure that the cooling air inlets are not covered or plugged at all.
Some iMacs have a round inlet vent under the hinge in the back (above the power connector)
Every few months, you can wipe across that vent to clear it.