Thanks for the purchase! Better multi-monitor support is on the agenda. You can eliminate the pillarboxed art/black bars by selecting "Zoom in to art to fill screen" at some sacrifice of resolution. There's also an option to prefer landscape-orientation artwork, which is really a filter to only select art with a width:height ratio of >1.3.
Would be nice to have an option for different art pieces on different spaces as well. For example, I use 4 spaces on my primary display and 1 space on the external one and I'd be delighted if I had 5 different art pieces shown.
