I have reason to think that ultimately these two didn't get along very well. In particular, Radahn used his mother's heritage against itself to enable his father's. Their brand of sorcery was tied to the movements of the stars and he used his gravity magic to stop their movements. Ranni could not achieve her destiny so long as they were halted.
I'd say that this pic represents a time before Radahn halted the stars, a time when he respected both of his parents, a time when Rannela & Radagon were still together, or something like that.
