It sounds like you are trying to use the image outside of a data region (Table, Matrix, List) which has a data set assigned to it. I assume this is a single image and not a different image per row in the data set? IF so you can do some thing like this:

=Lookup("[Key Value to Search On]", Fields![Key Field].Value, Fields!photpath.Value, "[Dataset Name]")

Or

=First(Fields!photopath.Value, "[Dataset Name]")