We have this functionality in our
Nodes add-on. We kept it separate since nodes information was used elsewhere, and this was mainly for 'user generated' covers, rather than something coming from the administrator. If users could set their own backgrounds for nodes, then this would make sense, but ultimately we felt users would not want to set images on nodes, that type of customization (imo) should be left to the administrator.
ETA: just noticed this was for XF1. Same story with XenForo 1 but in this case we put the customization of nodes into our theme UI.X. We thought this was wrong, so switched it in XenForo 2. Long story short, we do not have support for Node background images without UI.X theme.