If you used the Advanced edit mode with ESXi-Customizer 1.0 or 1.1 and got a "Corrupt boot image" message in ESXi (either when booting the customized ISO or after having installed with it) ... this was caused by a corruption of the OEM.tgz file while re-packaging it.
It was very hard to find a Windows version of tar that produces tar archives which are fully compatible with ESXi. But (I hope) I have finally found one: a Windows port of busybox. Since ESXi uses busybox, too, this should guarantee maximum compatibility. If you ever wondered what a Windows port of busybox could be good for ... now you know ;-)
Please update to version 1.2 that incorporates this fix, and let me know if you are still struck by this bug! Please download it from the project page!