How to obtain Disk Image of OS X Beta?
I'm afraid a DMG isn't downloadable.

I think it may be possible by the following steps: (haven't tried it myself yet)


Create a new disk image through Disk Utility at a size large enough to fit a system (about 40G should be good, you can shrink it later)
Mount the disk image
Using the installer downloaded from the App Store, install El Capitan Beta on the mounted disk image
Eject and export the disk image.