Can not output image title and description for TYPO3 extension DCE images

I created in TYPO3 extension DCE an image field that allows to input an image with caption and title field in the backend. But I couldn't access the value of these fields to input in the alt-tag in source. No values are shown by f:debug, how can I access these values and output?

The fastest solution would be having a look into the Class Reference of TYPO3\CMS\Core\Resource\FileReference. There are many getters for the properties. They access (and are allowed to access) the mergedProperties-array. In your special case getDescription() is, what you are looking for.

So for getting description a simple {field.imageDesktop.{i}.description} should work (if field.imageDesktop.{i} is a FileReference).