I was running into an issue at the same place similar circumstances ... although I couldn’t find any docs…. I found short file names on the images worked better for some reason….???
Example: win7hii.tbi or 1.tbi work,
My limit was this…
Anything over 7 characters failed. I believe this has to do with the maximum characters allow in the path… Depending how deep your file structure is will limit the URL or UNC you can use on the file name itself. So when the template calls imagew it can’t find the image file.. So thatsall you get and the process ends.
My structure was http://XXXXXXXXX.XXXX.XXXXXXXXX.com/images/Windows7/winimage.tbi failed evertime.
http://XXXXXXXXX.XXXX.XXXXXXXXX.com/images/Windows7/win7hii.tbi worked Just under the max path...
Or I could have remove the domain suffix at the time if knew that….
http://XXXXXXXXX/images/Windows7/winimageICanUseMoreHereNow.tbi failed
I also remove the map drive step to the image as that also breaking the process..
I also noticed this in packaging later, when the fully qualified names are used the dependent files end up error HTTP:// error 404, I can chop it to http://Server/packages/package and the subdirectories in package folder are now available… but the limit is riding pretty close still…
My guess this is an IIS issue as with my experience with apache web servers your limited to 3 directories deep on a web server…
You can certainly try the short image file name and see if imagew start.. if it does its finding the file! 
