There are environments, where users have the isolated vCenter server and have to manage t he cluster using the custom Image.
vCenter Server 7.0
vCenter server 8.0
Using a Isolated vCenter server where the LCM depot cannot sync with the Broadcom website to download the image manually.
The image would not show under the image drop down, unless the the ESXi offline bundle is not uploaded on the Lifecycle manager depot.
Go to Lifecycle Manager --------> Image Depot -------------> Action -----------> Import Image -------------------> Upload the offline bundle for the ESXi latest version where you want to upgrade your hosts to.
Refresh the vCenter server page to see that under the image depot.
Go to the Updates tab of the cluster and select Edit ---------Image now you will see the new version available.
For custom ISO you can directly upload the custom ISO that will show the Vendor Addons included. Else you can upload the standard image and add the hardware vendor addons manually.