Wallpapers that were selected for each MX release except MX 19 can be installed from the MXPI Popular apps. MX 19 wallpapers are included when MX 19 is installed. They are in /usr/share/backgrounds
Image galleries linked below include some very nice community submissions that were not selected to be included on the MX ISO. No account is needed to view or download these images.
MX 17 & 18 Community Submissions
MX 19 Community Submissions
See Wallpaper by Auro Kumar Sahoo