That global variable imloadfunc is s switch that tells which function to use to read images. If it is set to 'pgma_read', then it'll use that function. In any other case, it'll use the standard imread function.
It's a bad way of programming, there a much better mechanism in matlab for this such as getpref. Hopefully, that global variable is set by whatever starting code there is as if it does not exist your linked function will error.
If it's not set, you can always do:
global imloadfunc;
imloadfunc = ''; %to use default imread
at the command prompt.
