Because I am an author who creates written descriptions from scenes that are pictures in my head (but are they really?), I find this question most intriguing, and am probably biased as to an answer. The argument must begin with disregarding the theory that whatever we view in our heads is a picture itself. This is not strictly true because it ignores visibly imaginative and constructive people like myself, but, to make the question more equal on both sides, what may be rule for some people must become exception in this instance.
Let us start by the assumption that a picture is a physical entity and nothing else. A picture must exist in the world; it has to exist outside of mental conceptualism, whilst also providing evidence – also called ‘coherence’ – for what we know as a ‘picture’ in our mental ideas. In this way, a picture, painted or drawn or even crafted in another, less traditional format, is a object tied to principles of realism: something which exists outside of a meaning, where it gives the meaning to itself. That is to say, the picture is a concrete object.
For Plato, on the other hand, having an existing picture does not mean that it is the true Form of that ‘picture’ substance. He was an anti-realist for the matter, and claimed that the objects we perceive in the world around us are ‘pale imitations’ of the real world of Forms that exist as an alter-realm accessed only by the soul. In this way, one could argue that a picture is not a picture at all, and the question becomes moot.
A description sometimes goes by the name of a ‘word-picture’, this is true. One of the main arguments against this being a true picture – aside from the obvious that typed text is not a picture in the following thought that we would use (this thought abandons picture-shaped poetry, et cetera) – is that a description in words is simply that: words. What this criticism is saying is that subjectivity is what keeps a description from being a true image.
Anyone can see, for instance, a picture in a storybook of a black-and-white cow. The colours are noticed, as is the expression of the cow and its position. In the description of the cow, however, there are no visual constancies: ‘the white cow with a black patch across its flank wore a half-smile as it chewed through its cud’. This may seem more specific than the ‘black-and-white’ (ironically, a description in itself as this is prose not art), but, when read to the masses, who are then asked to draw the cow, with focus on its hide-colours, expression and position, each person’s image will be slightly different from the same piece of text – and not only because their drawing skill is subjective, too.
On the other hand, the exact opposite could be argued: that a picture only comes to existence as a true picture by the use of the viewer’s eyes to ‘receive’ their own interpretation of the artist’s mind.
A picture provides another dimension for the viewer, because it includes the viewer’s opinion, subjective and not explicitly set out from the picture; for instance, a cross may be, for one person, a symbol of God’s love for the world, but mean, for another, a sign of war.
And, whilst a description in words is usually (this could be disputed by authors: that a true description in words allows the readers to develop their own pictures of the subject) explicit to its matter – ‘the grass was as green as a glowing emerald’ – a picture allows for much more. A picture is suggestive of its story and meaning, whereas a description may dictate it too fully.
Both, it could be argued, give a portrayal of real life; through the lens, though the picture is distorted, we see part of our own reality reflected back at us, crafting in the hands of the artist. The same could be said when reading a piece of description.
Indeed, both are works of art, and both should involve the nature of ex eiectum viewpoint.
 See the Philosophy of Language Coherence Theory