Your question presents an interesting problem, in that 'digital art' is difficult to define. That difficulty is not surprising: 'art' is difficult to define, never mind the 'digital' part of it. The definition of 'art' I favor is a dagger definition offered by James Joyce:
Art is the human disposition of sensible or intelligible matter for an esthetic end.
[From The Critical Writings of James Joyce, R. Ellsworth and E. Mason (eds.), 1959. I seem to have loaned away my copy, so I can't provide a page number, but my often-errant memory insists this definition was in the Paris Notebooks.]
(Note that the foregoing definition of 'art' is often liberally re-interpreted by the venal and unscrupulous to mean 'the human disposition of sensible or intelligible matter for a pecuniary, pedagogical or moral end'. None of those substitutions for esthetic are even approximately adequate for the goal, defining 'art'.)
For my purposes in this answer, the definition from Joyce must be supplemented to include 'artwork'. 'Art' (including 'artwork') is therefore
the human disposition of sensible or intelligible matter for an esthetic end; works realized by such disposition.
'Digital art', then, is the computer-assisted human disposition etc. Additionally, if 'digital art' is to be substantially differentiated from other kinds of art, the assistance of the computer must be non-trivial. Non-trivial computer assistance for the human disposition etc. must be in the production or presentation of the humanly disposed sensible or intelligible matter.
Further, because technology in some form has always been involved with the production or presentation of art, it is not enough for computer assistance to be simply non-trivial.
For example, the printing of a contemporary novel, or the production of an ebook, involves "non-trivial computer assistance", yet the novel so produced would not be aptly called 'digital art'.
So I'll recommend that instead of 'non-trivial' the computer assistance in the production or presentation of 'digital art' must be esthetically significant. And so I arrive at my working definition of 'digital art':
Art (and artwork, as already defined) produced or presented with esthetically significant computer assistance.
The 'opposite' of this category, that is, art produced or presented without esthetically significant computer assistance, may not be well served by just one name. However, a good effort toward finding 'just one name' will observe that the opposite of 'computer-assisted' is 'without computer assistance'. In turn, 'without computer assistance', or rather 'without esthetically significant computer assistance', means (in the vernacular) 'low-tech'.
The best I can come up with for a single word designating the opposite of what is called 'digital art', then, is
This categorizes all art not produced or presented with esthetically significant computer assistance. Completely, the definition of 'low-tech art' is
the human disposition of sensible or intelligible matter for an esthetic end without esthetically significant computer assistance; works realized by such disposition.
One antonym of 'low-tech art' is 'digital art'. Another must be 'high-tech art', and perhaps that last is a more accurate term for 'digital art'.
One advantage of using 'low-tech' and 'high-tech' (as opposed to, for example, 'digital') is that the terms remain viable even as technology advances: what is understood to be 'high-tech' today may soon change, even radically, without obsoleting the term itself. The term 'digital', on the other hand, suffers from senescence in contemporary use, being that it is defined variously as
- Expressed in discrete numerical form, especially for use by a computer or other electronic device: digital information.
a. Relating to or being a device that can generate, record, process, receive, transmit, or display information that is represented in discrete numerical form.
b. Relating to or being a service that provides information expressed in discrete numerical form: We subscribe to digital cable.
- Relating to or being a profession or activity that is performed using digital devices: a digital librarian; digital photography.
[digital. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved January 18 2016 from http://www.thefreedictionary.com/digital . Emphasis mine.]
Underlying all of these senses is the sense of 'numerical'. It is readily observable, however, that the contemporary understanding of 'digital' does not draw on the sense of 'numerical'. If it did, the opposite of 'digital art' would be