How to create a string item containing the URL of an existing image item

Setup, Configuration and Use Beginners
2

Not easily. The state of the Image Item is base-64 encoded so you can’t just provide the URL using the REST API to get the state of the Item. What you’ll have to do is write the state of the Item to $OH_CONF/html and convert the base-64 encoding back to the binary image format. Then HomeHabit can use http:<OH ip or hostname>:8080/static/<nameOfFile>.png.

See OpenWeatherMap Use Icon Channel as icon on Sitemap for a relevant example.