I am in the beginning states of setting up a WDS my self so I can not give you a direct answer about how the tasks work
What i can tell you is that when i booted from PXE and it refered to the image i placed in WDS, it brought me through all the prompts that i set up in my task sequence (still having problems getting applications to work)
as for your boot image not being valid, it may be possible that your boot image is corrupt or not created properly.
I followed this tutorial for most of MDT setup, I found it extremely useful
http://www.windowsnetworking.com/articles_tutorials/Deploying-Windows-7-Part1.html