In the update function, you would have to check the $input['hero_image'] and $input['picture'] variables if they are different from ''. If they are == '' (equal to), then you need to change your validator or at least its rules dynamically to exclude the 'hero_image' rule. I have this in my code at one point and have different $rules defined in my model for if the input field is present and for when it's submitted empty (= unchanged).

I just found another, way easier and cleaner way to do it:

Laravel provides the ->sometimes() validator modifier (link). After you define your validator, do this:

$validation->sometimes('hero_image', 'required', function($input)
{
    return !empty($input->hero_image); // If the submitted field is empty, return FALSE to NOT include this validation rule
});

$validation->sometimes('picture', 'required', function($input)
{
    return !empty($input->hero_image);
});

// THEN, call the $validator->passes() function
if($validation->passes())
{
    ...

You also need to remove the 'hero_image' rule from your Post model.

// Eat. Sleep. Code. Repeat.