Good afternoon,
I just signed up for smugmug to do some testing. Here's what I discovered.
If you set your Gallery Settings to Unlisted (Anyone with a link) then it should work.
Go to Gallery Settings and change Visibility to Unlisted (Anyone with the Link) and Access Anyone with a Link.
Next go the file you want to use as a texture:
Click the icon at the bottom that gives you the SHARE option.
Choose the GET A LINK option. This will open a page with several link options. I think the option will be ORIGINAL.
I've not fully tested this so choose whatever works best for you.
Click the COPY next to the format you wish to use. Then paste the link into the url section for the cached image scipt.
And that should do it.
I hope this will work for you as well.
Best,
Anhton