Knowing the weather forecast, the pad crews drained the water pipes or left them running to prevent them freezing and bursting.
With sub-freezing temperatures predicted, ground crews drained most of
the water pipes at the launch pad to minimize ice formation. Those
that could not be drained were left running overnight, and strong wind
gusts blew water onto pad structures where it subsequently froze.
Weather Channel, Weather's Role in the Challenger Accident
Also from Rogers Commission Report (linked below)
The freeze protection plan for the launch pad was
implemented, but the results were not what had been
anticipated. The freeze protection plan usually involves
completely draining the water system. However, this was not
possible because of the imminent launch of 51-L. In order to
prevent pipes from freezing, a decision was made to allow
water to run slowly from the system. This had never been done
before, and the combination of freezing temperatures and stiff
winds caused large amounts of ice to form below the 240-foot level
of the fixed service structure including the access to the
crew emergency egress slide wire baskets. Ice also was forming in
the water trays beneath the vehicle.
Rockwell (makers of the Orbiter) were super concerned about the ice flying around at liftoff and damaging the tiles.
As John Tribe, chief engineer for Boeing/Rockwell Launch Support
Services, told Popular Mechanics:
"I couldn't believe they came out of the MMT [Mission Management Team] meeting with a recommendation to launch. Based on the ice alone,
I thought it would be no-go. The ice was an unknown."
(See also the Rogers Commission Report page 115, etc.)
Consider this a partial answer, I can't tell where exactly on the pad that picture is. This picture I took shows that there are a lot of pipes on the pad. The quote from the report says " below the 240-foot level of the fixed service structure including the access to the crew emergency egress slide wire baskets" My picture shows the joint between the rotating and fixed service structures.