Only by making the background picture wider.
In the current version of TRichView, there is no way to specify to repeat a background picture only vertically. It is repeated both vertically and horizontally.
Sorry no. At least not in this Summer.
It may look simple, adding these options. But I do not like the idea of adding too many options in BackgroundStyle.
Even for adding vertical tiling, it would be 6 new options:
(fixed tiles | scrolled tiles) x (left-aligned | centered | right-aligned)
So I think it makes sense to change background only when we decide to rework it completely: changing a set of properties for background (separate properties for vertical and horizontal positioning, different image types, gradients, etc.)