Created attachment 8082 
Screenshot with dcross-exif.jpg applied as wallpaper.

If an image is selected for background and it is a photo taken in prortrait mode, digital cameras add EXIF tag 0x0112 adding an orientation information. This is taken into account but scaling factor is calculated before image is rotated.

Here is a description how issue can be reproduced. It requires ImageMagick, exiftool and bash as prerequisites:

<script lang="bash">
#!/bin/bash
fname=dcross
convert -size 3456x5184 xc:lightblue \
          -draw "line 3455,0 0,5183" \
          -draw "line 0,0 3455,5183" \
          $fname-portrait.jpg

convert -size 5184x3456 xc:lightblue \
	-draw "line 5183,0 0,3455" \
	-draw "line 0,0 5184,3455" \
	$fname-landscape.jpg

if [ -f $fname-exif.jpg ]; then rm $fname-exif.jpg; fi
exiftool -Orientation=6 -n -o $fname-exif.jpg $fname-landscape.jpg
</script>

Result are three files:
* dcross-landscape.jpg
* dcross-portrait.jpg
* dcross-exif.jpg

They may be selected as wallpaper using xfdesktop-settings or xfconf-query (channel xfce4-desktop with image-style=4);

The first two are properly centered and scaled if selected as wallpaper. The last one is displayed wrong.
* Rotation is ok;
* What is shown is displayed centered and with same width like dcross-portrait.jpg.
* But something like a viewport is applied, clipping image horizontally and vertically. See screenshot screenshot-dcross-clipped.png

OS is openSUSE LEAP 15.0

This looks like scaling is calculated before image is rotated.

This report is probably related to https://bugzilla.xfce.org/show_bug.cgi?id=13259 and https://bugzilla.xfce.org/show_bug.cgi?id=14644