In this context I suggest it means :-
"These images SUPPORT some of society's deepest fears and prejudices about people with disabilities"
However "undergird" is not a common word to use in the UK.
I even had to look it up in the dictionary!
tr.v. un·der·gird·ed or un·der·girt (-gûrt′), un·der·gird·ing, un·der·girds
To support or strengthen from beneath. "