I haven’t a clue about the source of this image because it’s the first time I’ve seen it in color! It may well, however, have been taken from the original publication about the tomb.
Ada Cohen in
The Alexander Mosaic: Stories of Victory and Defeat has a reproduction of the image in black and white on page 56 (although it is cropped at the bottom and only shows the horse and rider). The photo credit is: Figure 32. Drawing after a lost painting with battle scene. From the lunette of the back wall of the Kinch Tomb at Lefkadia. End of the fourth or first half of the third century B.C.. After Kinch 1920: pl. II.
In the text on page 54 she says:
There is also the testimony of an unfortunately now-lost painting discovered early in this century in the so-called Kinch Tomb, a vaulted Macedonian tomb near modern Naoussa. The painting decorated the lunette of the tomb’s back wall. Almost nothing of the painting remains today (traces of a horse’s tail only), but the initial short publication by the Danish excavator contained a lovely watercolor that is reliable insofar as the main pictorial elements are concerned, even if not in all detail (Fig. 32) (Note 11). The painting shows a Macedonian horseman drawn in profile and three-quarter views. He is richly clad in purple-red and blue and, like Alexander in the Mosaic, uses an animal skin as saddle. He is speeding against his target, a Persian enemy, in a manner reminiscent of Alexander in the Mosaic. The Persian is trying to curb the rider’s inexorable motion, proffering with both arms his shield against the enemy for protection. Paradoxically, this shield carries the so-called Macedonian star as decoration. The excavator dated the painting approximately to the turn of the fourth to the third century B.C., but a relatively recent reevaluation has placed it in the middle of the third century B.C., (Note 12) by relating it to another Macedonian tomb excavated at Dion by George Soteriades. The date of the latter, however, is itself debatable, variously assigned from the end of the fourth to the middle of the third century B.C. This tomb at Dion contained a marble kline decorated with a painting of an equestrian battle set against an empty pinkish-orange ground. In this battle Greeks seem again to fight against barbarians, if one can judge from the trousers of two of the three surviving riders.
(Note 11) See Kinch 1920: plates I, II, III, and IV (drawings and photographs of the painting); see p. 4 for Kinch’s dating of the tomb; Swindler 1929: 300; Gossel 1980: 170-78; Andreou 1988: 110-11; Andronikos 1964: 299-300; Rhomiopoulou and Touratsoglou 1971: 146-64; Rumpf 1953: 150; Robertson 1975: 571; and now Miller 1993: 13 with n. 73, 109-10. See Six 1925: 263-80 for the proposal that this is a copy of a painting of Antipater executed by Nikomachos.
(Note 12) Kinch 1920; for the later study, see Rhomiopoulou and Touratsoglou 1971: esp. 163-64. Brown 1957: 86 proposes too late a date in the later second century B.C. Stella Miller suggests a date early in the second century B.C. or very late in the third century for this highly problematical tomb and its painted horseman, assumed to represent the dead man in battle (Miller 1971: 111-13). See also Gossel 1980: 170-78 (not before the middle of the third century B.C., 177) and Miller 1993: 13, 109-10.
The bibliography has this for Kinch:
Kinch, K.F. 1920. “Le tombeau de Niausta. Tombeau Macedonien.”
Memoires de l’ Academie Royale des Sciences et des Lettres de Danemark, Copenhague, 7me Serie Section des Lettres. Vol. IV. No. 3. 285-88.
And that’s all I have.

I’ve seen a couple of other reproductions of the same image in black and white, including one in article by Markle, but no information is given whatsoever as to the source of the photograph.
Best regards,