c>>Maybe just "perception of attractiveness/beauty"?
c> Is a nimitta always related to visible forms/images? What about
c> meditators who work with breath?
Exactly, you have mentioned an important point - nimitta is in no way
limited to sight. It can manifest in any sensory channels.
The term 'perceptual image' is meant here in a multi-modal sense - for
example, at the time when you are musing about your favourite song,
you can notice the sound of it even on the busy street. 'Nimitta', as
Buddhaghosa writes, helps to recognize something. It is a collection
of sensory data peculiar to some class of objects or inner states.
The meditator, keeping in mind the perceptual image of jhana, can
notice even the subtle signs of it approaching - be it tactile
sensations or pecularities of visual perception.
In the sutta which you are translating, the person is so much looking
for sensual pleasure, that he is focused exclusively on the attractive
features, ignoring anything else - unattractive features, causes and
consequences of actions.
For example, modern cars fan looks at the latest car, being attuned to
the attractiveness of its lines, and immediately wants to buy it and
have it as a part of 'self'.
In such cases a person does not have a 'perception of attractiveness'
- 'attractiveness' is not an external object which is perceived. It is
more correct to say that the person has attunement to the perceptual
image of attractiveness, or 'perceptual attunement' to attractiveness.