I’m not sure there’s quite the dispute or divide you imagine (!) between the Romantics and the Orthodox. You’re right, of course, that many Fathers, both ancient and modern, often warn against the distracting dangers of imagination. To pick but one example, St Maximos the Confessor writes, “An intellect which dwells in imagination is unable to advance through contemplation to those intelligible realities cognate with it.”
If you take a look at the glossary of the Philokalia (from which this quotation is taken), you’ll find, however, that the word here rendered as “imagination” is the Greek phantasia, that is, “fantasy” or simply “fancy”.
Whether he can be taken as speaking for all the Romantics, I can’t say, but the poet, philosopher, and literary theorist S. T. Coleridge distinguishes quite emphatically between imagination and fancy in his Biographia Literaria, Ch. 13. At least on a terminological level, he agrees with the Fathers of the Philokalia in seeing the second as at best second rate. In case you’re interested, I discuss this at some length in my book The Form of Transformed Vision: Coleridge and the Knowledge of God.
The phantasia disparaged in the Philokalia, and among Orthodox authorities in general, is not the “imagination”, in STC’s usage, but rather “fancy”, which (says Coleridge) is “no other than a mode of Memory emancipated from the order of time and space…. Equally with the ordinary memory it must receive all its materials ready made from the law of association” (Biographia Literaria [Princeton, 1983], Vol. I, 305).
The “imagination”, by contrast is something quite different. It comes in two forms, primary and secondary. The first is “the living Power and prime Agent of all human perception” and “a representative in the finite mind of the eternal act of creation in the infinite I AM” (BL, ibid., 304). To use more characteristically Orthodox language, it’s our cognitive participation in the Logos, “the light of men” (John 1), whereby one is enabled to see the inner logoi of things. The secondary imagination, on the other hand, “co-existing with the conscious will”, presupposes the “light” of the primary—STC calls it “an echo of the former”—but it adds a dimension of intentionality as it “struggles to idealize and to unify” (304).
As I see it, the key—or at least a key—difference between Orthodox phantasia and Coleridgean imagination is that the latter is necessarily neptic: it depends on a degree of wakefulness, and its aim, in its secondary form, is to assist and extend the creative work of God. I can’t help but think in this connection of J. R. R. Tolkien’s “sub-creation”, the art of making “according to the law by which we are made”.