First make sure that you are logged into the correct lab. There is a drop-down in the top left that lets you select your lab.
If you are in the correct lab but not seeing options when trying to select the image you may need to enable the images in the lab configuration. You can navigate to Marketplace images and select the appropriate image or Azure compute gallery if using a custom image. Once the image is selected you can enable it, make sure to hit Apply in the bottom left to save.
After going back to the Labs portal you should be able to select any image that was added to the lab configuration.
If this doesn't work for you please let me know and we can work with you directly to further investigate.
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