I encountered this issue on Ubuntu 19.04 Beta. I downloaded the iso today, and installed the system just today.

I confirmed that the issue below happens on Ubuntu Gnome, Ubuntu Gnome on Wayland, Vanilla gnome session on Xorg, and Vanilla gnome session on Wayland.

I set a background image, and the background goes white when I set my display setting to 3840x2160 with 200% scaling. When I set my background to a solid color, it displays that color fine. The issue happens when I use a jpeg with resolution of 3840x2160 with 200% scaling.

No issue when scaling is set to 100% with the same resolution.

Below is journal log when I reproduce the problem.

Apr 17 15:20:51 prudentubuntu systemd[1]: Started Firmware update daemon.
Apr 17 15:20:51 prudentubuntu PackageKit[1266]: uid 1000 is trying to obtain org.freedesktop.packagekit.system-sources-refresh auth (only_trusted:0)
Apr 17 15:20:51 prudentubuntu PackageKit[1266]: uid 1000 obtained auth for org.freedesktop.packagekit.system-sources-refresh
Apr 17 15:20:52 prudentubuntu gnome-shell[2253]: [AppIndicatorSupport-DEBUG] Registering StatusNotifierItem :1.76/org/ayatana/NotificationItem/software_update_available
Apr 17 15:20:52 prudentubuntu gnome-shell[2253]: [AppIndicatorSupport-FATAL] unable to update overlay icon
Apr 17 15:20:52 prudentubuntu gnome-shell[2253]: [AppIndicatorSupport-FATAL] unable to update overlay icon
Apr 17 15:20:53 prudentubuntu PackageKit[1266]: refresh-cache transaction /68_bdbeeadd from uid 1000 finished with success after 1806ms
Apr 17 15:20:53 prudentubuntu gnome-software[2725]: not handling error invalid-format for action refresh: JSON data must be UTF-8 encoded
Apr 17 15:20:54 prudentubuntu gnome-software[2725]: hiding category audio-video featured applications: found only 0 to show, need at least 9
Apr 17 15:20:54 prudentubuntu gnome-software[2725]: failed to get featured apps: no apps to show
Apr 17 15:20:55 prudentubuntu gnome-software[2725]: Only 0 apps for popular list, hiding
Apr 17 15:20:56 prudentubuntu PackageKit[1266]: resolve transaction /69_cdcbbcad from uid 1000 finished with success after 3169ms
Apr 17 15:20:56 prudentubuntu PackageKit[1266]: resolve transaction /70_ccdadbbc from uid 1000 finished with success after 368ms
Apr 17 15:20:57 prudentubuntu gnome-software[2725]: hiding category productivity featured applications: found only 1 to show, need at least 9
Apr 17 15:20:57 prudentubuntu PackageKit[1266]: search-file transaction /71_cbadaeab from uid 1000 finished with success after 647ms
Apr 17 15:20:57 prudentubuntu PackageKit[1266]: get-details transaction /72_edeccdbb from uid 1000 finished with success after 329ms
Apr 17 15:20:58 prudentubuntu gnome-software[2725]: Failed to load snap icon: local snap has no icon
Apr 17 15:20:58 prudentubuntu gnome-software[2725]: Failed to load snap icon: local snap has no icon
Apr 17 15:20:58 prudentubuntu gnome-software[2725]: Failed to load snap icon: local snap has no icon
Apr 17 15:20:58 prudentubuntu gnome-software[2725]: Failed to load snap icon: local snap has no icon
Apr 17 15:21:36 prudentubuntu gnome-shell[2253]: Object Meta.Background (0x55cdf7640d10), has been already deallocated — impossible to access it. This might be caused by the object having been destroyed from C code using something such as destroy(), dispose(), or remove() vfuncs.
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: == Stack trace for context 0x55cdf4323280 ==
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #0   55cdf99c0128 i   resource:///org/gnome/shell/ui/background.js:722 (7f1872f08430 @ 22)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #1   55cdf99c00a0 i   resource:///org/gnome/shell/ui/tweener.js:106 (7f18880c8940 @ 37)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #2   7fff0824c1e0 b   resource:///org/gnome/gjs/modules/tweener/tweener.js:208 (7f18880d0670 @ 54)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #3   7fff0824c330 b   resource:///org/gnome/gjs/modules/tweener/tweener.js:342 (7f18880d0700 @ 1742)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #4   7fff0824c3e0 b   resource:///org/gnome/gjs/modules/tweener/tweener.js:355 (7f18880d0790 @ 100)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #5   7fff0824c470 b   resource:///org/gnome/gjs/modules/tweener/tweener.js:370 (7f18880d0820 @ 10)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #6   7fff0824c4f0 I   resource:///org/gnome/gjs/modules/signals.js:128 (7f18880cb820 @ 386)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #7   7fff0824c5a0 b   resource:///org/gnome/shell/ui/tweener.js:238 (7f18880cb3a0 @ 159)
Apr 17 15:21:36 prudentubuntu org.gnome.Shell.desktop[2253]: #8   7fff0824c640 b   resource:///org/gnome/shell/ui/tweener.js:209 (7f18880cb280 @ 15)
Apr 17 15:21:36 prudentubuntu gnome-shell[2253]: g_object_run_dispose: assertion 'G_IS_OBJECT (object)' failed