Leaving aside their interpersonal function of valuation, the meanings 'ideal' and 'real', like the meanings 'idea' and 'thing', are construals of experience, and, as such, are ideational, not textual, with regard to metafunction.
Halliday & Matthiessen (1999: 7):
The ideational metafunction is concerned with construing experience — it is language as a theory of reality, as a resource for reflecting on the world.
'Real' refers to the material order of experience: things (and other phenomena); whereas 'ideal' refers to the semiotic order: ideas of things (and other phenomena).
As levels of abstraction, 'ideal' is higher than 'real', and positioning expressions of 'the ideal' above expressions of 'the real' in images is a visual means of representing that ideational relation.
The textual metafunction, on the other hand, is concerned with organising the ideational and interpersonal, as through giving different degrees of textual prominence — e.g. thematic or informational — to ideational and to interpersonal meanings.
Halliday & Matthiessen (1999: 7-8):
The textual metafunction is an enabling one; it is concerned with organising ideational and interpersonal meaning as discourse — as meaning that is contextualised and shared. But this does not mean processing some pre-existing body of information; rather it is the ongoing creation of a semiotic realm of reality.
If it is true that the top portion of an image has a textual function in the construction of an image as message, this is quite distinct from the types of ideational meaning that are likely to be given textual prominence by such means.
Halliday & Matthiessen (2004: 85):
Initial position in the English clause is meaningful in the construction of the clause as message; specifically, it has a thematic function.