If Walton is truly using this word in a painterly sense, then the meaning is unusual,
centered on the concept of harmony or proportion, thus not at all equivalent to the
action of "maintaining" one would expect without the attached caveat. Here are its
painterly connotations as represented by the Oxford English Dictionary:

a. In Painting. orig. The maintenance of the proper relation between the representations
of nearer and more distant objects in a picture; hence, in more general sense, "the
proper subserviency of tone and colour in every part of a picture, so that the general
effect is harmonious to the eye" (Fairholt); the maintenance of harmony of composition.

• 1715 J. Richardson The. Painting 224 The Composition is not to be justifyed . . .
  the Groups are too Regularly placed, and without any Keeping in the Whole, that is,
  they appear too near of an Equal Strength.
• 1762 Goldsm. Cit. W. lv, [Parodying art slang] `What do you think, sir, of that head
  in the corner, done in the manner of Grisoni? There's the true keeping in it.
• 1768 W. Gilpin Upon Prints 20 Keeping then proportions a proper degree of strength
  to the near and distant parts, in respect to each other.
• 1780 Johnson Let. to Mrs. Thrale 1 May, There is contour, and keeping, and grace,
  and expression, and all the varieties of artificial excellence.
• 1792 Resid. France (1797) I. 87 Some mixture of splendour and clumsiness, and a want
  of what the painters call keeping.
• 1809 Mar. Edgeworth Absentee x, In Lady Clonbrony's mind, as in some bad paintings,
  there was no keeping; all objects, great and small, were upon the same level.
• 1859 Gullick & Timbs Paint. 172 The keeping and repose in this cartoon are inimitable.

b. generally. Agreement, congruity, harmony.

• 1819 Hazlitt Eng. Com. Writers vi. (1869) 153 There is the exquisite keeping in the
  character of Blifil, and the want of it in that of Tom Jones.
• 1870 Lowell Study Wind. 406 For wit, fancy, invention, and keeping, it [the Rape of
  the Lock] has never been surpassed.

c. Phr. IN OR OUT OF KEEPING WITH—in or out of keeping (within) or out of keeping
(with): in or out of harmony or agreement (with).

• C. 1790 Imison Sch. Art II. 59 In what respect it is out of keeping; that is, what
  parts are too light, and what too dark.
• 1806 F. Horner Let. in Life vii. (1849) 175 They were so in keeping with the whole
  that the prevailing tone was..never interrupted.
• 1824 Miss Ferrier Inher. xlvii, To use an artist phrase, nothing could be more in
  keeping with the day than the reception Miss S. met with.
• 1830 Blackw. Mag. XXVII. 310 It is in `fine keeping', as the phrase is.
• 1841 Lever C. O'Malley iv. 24 His own costume of black coat, leathers and tops, was
  in perfect keeping.
• 1841 Myers Cath. Th. iii. v. 14 Such an utterance of Truth would . . . be out of keeping
  with our present condition in the flesh.
• 1878 Bosw. Smith Carthage 26 Indications . . . in thorough keeping with the view we
  have taken.