I think you might want to change your parameters from:
%{"image" => %{"category_id" => 1, "description" => "This is a test image", "image" => "mylovelywife.png", "name" => "i1"}}
to:
%{"image" => %{"category" => %{"id" => 1}, "description" => "This is a test image", "image" => "mylovelywife.png", "name" => "i1"}}
cast_assoc expects the embed to have nested params.
