Try this first, you may be passing a Null Model:
@if (Model != null && !String.IsNullOrEmpty(Model.ImageName))
{
<label for="Image">Change picture</label>
}
else
{
<label for="Image">Add picture</label>
}
Otherise, you can make it even neater with some ternary fun! - but that will still error if your model is Null.
<label for="Image">@(String.IsNullOrEmpty(Model.ImageName) ? "Add" : "Change") picture</label>