Permissions errors when using launchd to run rsync between local drive and afp network drive

I am trying to sync a local directory to my NAS that is mounted via the afp protocol as a drive. I am using rsync. When I run the command from the terminal, it works fine. However, when I try to run it from launchd, it always fails.
This is my rsync command:

rsync -av --delete /Users/self/Pictures/ /Volumes/nas/Images

This is the plist file that contains the above command. It is located in ~/Library/LaunchAgents with user, groups, and permissions equal to the other (working) plists in that directory.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>rsync_picture_to_nas</string>

    <key>ProgramArguments</key>
    <array>
        <string>rsync</string>
        <string>-av</string>
        <string>--delete</string>
        <string>/Users/self/Pictures/</string>
        <string>/Volumes/nas/Images</string>
    </array>

    <key>StandardErrorPath</key>
    <string>/Users/self/.log/rsync_picture_to_nas_err.log</string>

    <key>StandardOutPath</key>
    <string>/Users/self/.log/rsync_picture_to_nas_out.log</string>

    <key>WatchPaths</key>
    <array>
        <string>/Users/self/Pictures</string>
    </array>
</dict>
</plist>

It always has complete permissions failure. The exact permissions errors will depend on whether I am running it from scratch or have previously run it successfully from the command line. Suffice to say, it is totally incapable of doing anything. It can’t create or modify any files on the NAS at all.

Here is a sample error:

rsync: recv_generator: mkdir "/Volumes/nas/Images/Lightroom Library.lrlibrary" failed: Operation not permitted (1)
*** Skipping everything below this failed directory ***