Some thoughts on the requirements
If the picture is floor to ceiling height and largely black/ dark,
1. Subtle highlighting of the area around the main lighting strike (from top to bottom), otherwise relatively uniform lighting of the entire piece (left to right).
2. No light scallops, or spots of light on the picture itself.

In my mind, that could be accomplished using wall washing with light bars (for example LED eW PowerCore MX with a Color Rendition Index > 80).

Since there will be individual bars which are ~ 1 ft in length, the portion highlighting the main lightning strike could be on a separate circuit from the rest and the entire array of bars controlled so that the important part gets more lighting.

There will be a small gap between joined bars.

The entire setup could use some ray tracing help and require experimentation.