Had the same problem
'http://www.binarymoon.co.uk/2009/10/timthumb-wordpress-mu/ '
CANNOT be used that fashion....
Create a Custom Filed Named "thumb" (no quotes) next, each post you would like the image to appear, try adding this
'http://www.binarymoon.co.uk/wp-content/blogs.dir/1/files/2010/01/myimage.jpg'
Now for the really fun stuff --- you must obviously change the month and year as they change AND for each blog, the number /1/ for example must be changed to reflect each blog e.g /2/, /3/, etc
Give it a whirl and see what happens...
If you get stuck or not certain how to make a "Custom Field Entry" let anyone know, the moderators or I could help you on that...
A rather headbanger to figure out not to mention, that all your other users will need to get used to it as well.
I use "Admin Ads" and post the current link at the top of the page with some brief instructions (of course I already created the custom field for them), they only have to add the
'http://www.binarymoon.co.uk/wp-content/blogs.dir/1/files/2010/01/myimage.jpg' into the value field...
Good Luck