Can't Access Image URL on Single Page
You left off the .first().

{% set notFound = craft.entries.section('page404').first() %}


Without that, the Element Criteria Model will default to .find().


first - Returns a single Entry Model.
find - Returns an array of Entry Models.


It doesn't matter this this is a Single section! All Element Criteria Models have the same behavior, whether they're fetching data from a Single, Channel, or Structure.