Here's a workaround:
1. Create a workflow, including the "Watermark PDF Documents" action, save it as a workflow.
2. Show the contents of the saved workflow, find the file "document.wflow" inside the "Contents" folder, open it with a word processor, e.g TextEdit or BBEdit or Microsoft Word
3. Find the parameter block of the "Watermark PDF Documents" action, it should look like below:
<key>ActionParameters</key>
<dict>
<key>angle</key>
<real>57.379999999999995</real>
<key>onePDF</key>
<false/>
<key>opacity</key>
<real>0.72282608695652173</real>
<key>overUnder</key>
<integer>1</integer>
<key>scale</key>
<real>1.4574821967095799</real>
<key>xOffset</key>
<real>34.5</real>
<key>yOffset</key>
<real>23.469999999999999</real>
</dict>
4. Add a key-value pair for "fileNames" into the parameter dictionary, so that it looks like this:
<key>ActionParameters</key>
<dict>
<key>angle</key>
<real>57.379999999999995</real>
<key>fileNames</key>
<array>
<string>/Volumes/Storage/Media/Pictures/SleepingCat.gif</string>
</array>
<key>onePDF</key>
<false/>
<key>opacity</key>
<real>0.72282608695652173</real>
<key>overUnder</key>
<integer>1</integer>
<key>scale</key>
<real>1.4574821967095799</real>
<key>xOffset</key>
<real>34.5</real>
<key>yOffset</key>
<real>23.469999999999999</real>
</dict>