Plugin doesn't load / update if using jpg or gif image for background

[lectrode]

This issue can be recreated in 2 ways:

1. Set panel background to a jpg or gif image (Panel pref -> Appearance) AND/OR
2. Set the background image directly in the plugin settings

After this, the dockbarx plugin does not update it's background. Additionally, upon reloading the panel, the dockbarx plugin is no longer there.

Tested with: dockbarx (0.92-1 - latest manjaro package) xfce4-dockbarx-plugin (0.5-1 - latest manjaro package) xfce4-dockbarx-plugin-git (49.a2dcb66-1 - latest AUR package)

Tested those against both: xfce4-panel (4.12.2-1 - latest manjaro package) xfce4-panel-gtk3 (4.13.3-2 - latest manjaro package)

=Current workaround= It works as expected if you use a png image instead. I have not tested other image formats.

[TiZ-HugLife]

This is pretty much entirely because I presumed that the panel background would always be a PNG file. It's lossless and supports translucency, so I couldn't conceive why anyone would try to use an indexed lower-color GIF or a lossy JPG. It's not my place to question your usecase though, so I'll look into how xfce4-panel handles it and then try to handle it similarly. Thanks for the report.