It should be stated that MODX should be updated to the latest version - if the client's server doesn't support it they should request that the server be upgraded to support modern software OR seriously consider changing web hosts.
That having been said:
Make sure the assets/images directory has the correct permissions & owner. I believe the user:group should be [user]:www-data, or whatever your server uses for the group that the web server's process belongs to.
You would want to issue a command something like:
chown -R [user]:www-data /path/to/your/install/assets/images
Take special care whenever you issue a command like chown recursively.
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