If your own photo library is accessible over the internet using a web browser (cloud stored maybe) then yes, just copy the internet address (URL) of the picture from your browser address bar into the input field in Roon.
Note: Not all photos/pictures from the internet work by this method, some sites “hide” the actual picture URL in their site code in ways Roon can’t understand.
If you want to use a locally stored picture from your device, a tablet or phone can’t do that – please try and use a PC (Windows or MacOS) instead.
See also (and vote if you like the idea):
The picture is a hyperlink to my feature suggestion.