Enter an URI of a custom user image and operating system type.
Your custom image has to be available into the same storage account in which you are going to create agent nodes.

e.g. https://jenkinsarm.blob.core.windows.net/system/Microsoft.Compute/Images/vhds/jenkinsvm5-osDisk.c68efe70-4b40-450e-a8b6-3d10b7d1665f.vhd

If you hit the storage scalability limits for your custom images on the storage account where the VHD resides, you should consider using the agent's temporary storage or copy your custom image in multiple storage accounts and use multiple VM templates with the same label within the same agent cloud.