Just for giggles I tried it on Mate last night and yeah it installed fine and worked great up until I hit that Apply button...it was a hoot, it actually tried to apply the paper but only like a quarter of it laid over the previous wallpaper.

Source: https://git.devuan.org/greenjeans/vuupaper

Looking good I think, re-worked the first-run dialog too so it's much better.

1. Fixed the alphabetizing issue, case-insensitive now. Real forehead slapper that, i'm usually a stickler for sorting, guess I got excited and rushed it out a bit. Even numbered files weren't right. All better now.

2. Fixed first run, it will now default to /usr/share/backgrounds if you click Cancel. And that's where the new thought came from, I should just label that button "Default" and add to the first run dialog: " Click Default below to use the standard system folder /usr/share/backgrounds, or click Choose Directory to select a different folder"

3. Also related to first-run, when you hit the Choose Directory button, it now opens the file-chooser to /usr/share/backgrounds, as possibly someone might have specific folders within that directory they want to choose instead of the main directory. But of course a user can navigate from there wherever they want.

4. I did a LOT of messing with gtk, if you've ever tried it you know how many re-compiles that takes, it can be weird and finicky at times but I suppose there's reasons for that. Long story short I tightened up all the padding/spacing, it now has a nice grid of 12 displayed at default size with rows of 4 with room left for the scroll bar and a little breathing room at the right side and bottom. I re-claimed a lot of real estate that way, only 10 px in height saved but I reduced width by 120 px.
Click the thumb below for a larger image.

Up on gitea now: https://git.devuan.org/greenjeans/vuupaper

This is the first version, still might have some small bugs and surely could use some polishing, but it seems pretty stable
and very useful even in multi-monitor setups. Some nice features and workflow too, check it out when you have time! To compile yourself:

gcc -o vuupaper22 vuupaper22.c `pkg-config --cflags --libs gtk+-3.0` -Wall -O2

From the readme:

VuuPaper, a simple wallpaper manager/setter for X.

This app was inspired by Nitrogen and is similar in look and feel, this app is built with C and gtk3
and uses the excellent xwallpaper for the backend. Very small and fast, less than 40 kb for the binary.

On first run a dialog is opened that offers quick instructions about the --restore function, and
a Choose Directory button that opens the file-chooser for you to select your default wallpaper
folder, and then opens the app with thumbnails of your images ready to apply. You can add additional
wallpaper folders by using the Add Folder button, to the right of that is a drop-down menu from
which you can select what folder you want to use. The Style menu lets you choose what mode to
apply the wallpaper with (i.e. zoom, center, maximize etc.) The BG Color button lets you choose
what background color is behind your image. On the upper right is the monitor selector drop-down
menu, you can choose from All Monitors (same image/style on all), Span (one image spanning all),
and after that will be listed all monitors connected. And of course the Apply button to use after
you have selected a monitor, wallpaper, and style.

At the top is the infobar which gives you the full path to the wallpaper currently applied, for
multi-monitor setups the monitor drop-down selector also toggles the infobar if you have different
papers applied on different monitors, so you can always know what's currently on each display.

To make your wallpaper choices restore after reboot, you need to add it to your autostart file
similar to the way other wallpaper apps do:
"vuupaper --restore &"
In Openbox this is usually ~/.config/openbox/autostart but there are many ways to do it, consult
the docs for your system/WM/DE to find out what's best.

I thought this might be something you'd like Can I get you to test the beta when it's ready?