The only things that I know are safe to delete before doing an acronis image....
I run Acronis from BartPE, it's perfect... because I can fully get rid of many things you can't otherwise..
Dump all filels from all Temp Folders, Temp Inet Folders, History folders, Cookies, etc.
You can safely dump the files from Prefetch folder
If there are other folders for Apps that you can get rid of cause you don't need them, then by all means do so. The smaller you can make the image the better.
I then run Defraggler on the C: drive from BartPE
Then make my image.
The reason I don't go any farther than that, is cause i don't know what will get recreated and what will not.