Android update and android mod I want to create a custom boot image for my Android. When I will update the System OTA I will lose my mod?
That very much depends on the OTA and what it includes: it might only replace/add some files, but it could as well replace an entire partition (usually /boot and/or /system).

So yes, an OTA system update might overwrite your "custom boot image" – though not all OTAs will do so.