private bool _fired;

[Test]
public void CheckVisiblityEvent()
{
    _fired = false;
    ImageGroupListItemViewModel vm = ImageGroupListItemViewModelFactory.Build();
    vm.VisibilityChanged += new ImageGroupListItemViewModel.ImageGroupVisibilityChangedEventHandler(vm_VisibilityChanged);

    vm.Visible = true;
    Assert.IsTrue(_fired);
}

void vm_VisibilityChanged(object sender, ImageGroupVisibilityEventArgs args)
{
    _fired = true;
}

This is a bit sloppy. Instead, we could improve on this by using and anonymous method:

[Test]
public void CheckVisiblityEvent()
{
    bool fired = false;
    ImageGroupListItemViewModel vm = ImageGroupListItemViewModelFactory.Build();
    vm.VisibilityChanged += delegate(object sender, ImageGroupVisibilityEventArgs e)
    {
        fired = true;
    };
    vm.IsVisible = true;
    Assert.IsTrue(fired);
}

More compact and easier to follow. Plus, the test code is all in one place. We can improve this even more with a lambda expression:

[Test]
public void CheckVisiblityEvent()
{
    bool fired = false;
    ImageGroupListItemViewModel vm = ImageGroupListItemViewModelFactory.Build();
    vm.VisibilityChanged += (s, e) => fired = true;
    vm.IsVisible = true;
    Assert.IsTrue(fired);
}