NOTE: See (also) a corrected/improved answer in the question itself.
Following solution worked for me:
Open a terminal and backup com.apple.dynamic_pager.plist which you're going to change in a second:
$ cd /System/Library/LaunchDaemons
$ sudo cp com.apple.dynamic_pager.plist{,_bak}
convert binary plist to xml:
$ sudo plutil -convert xml1 com.apple.dynamic_pager.plist
and open it with your favorite text editor
$ sudo vim com.apple.dynamic_pager.plist
it'll look something like this:
1 <?xml version="1.0" encoding="UTF-8"?>
2 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
3 <plist version="1.0">
4 <dict>
5 <key>EnableTransactions</key>
6 <true/>
7 <key>HopefullyExitsLast</key>
8 <true/>
9 <key>Label</key>
10 <string>com.apple.dynamic_pager</string>
11 <key>OnDemand</key>
12 <false/>
13 <key>ProgramArguments</key>
14 <array>
15 <string>/sbin/dynamic_pager</string>
16 <string>-F</string>
17 <string>/private/var/vm/swapfile</string>
18 </array>
19 </dict>
20 </plist>
In line 17 modify /private/var/vm/swapfile (e.g. /Volumes/partition2/swapfile), save and close your editor (":x" will do both in vim).
convert the plist file back to binary:
$ sudo plutil -convert binary1 com.apple.dynamic_pager.plist
After rebooting your Mac you should find the swapfiles in the directory you specified.
If you run into any problems you can restore the backup you created in the first step with:
$ cd /System/Library/LaunchDaemons
$ sudo cp com.apple.dynamic_pager.plist{_bak,}
Best Answer
Does this help? Preview is not really that scriptable.
For what it's worth, Automator can do image manipulation more easily, it also has some PDF actions.
If you're wanting to do image manipulation with Applescript, what you're really looking for is Image Events, not Preview.