It is a function in the nnet.internal.cnnhost package directory. Its name is not resizeImage2D but nnet.internal.cnnhost.resizeImage2D. As the "internal" part of the package name indicates, you should not write code depending on the behavior or existence of this function in a future release.
According to the which function, this function is built-in and so you cannot read the code.
>> which -all nnet.internal.cnnhost.resizeImage2D
resizeImage2D is a built-in method% static method or package function