Small fix for Images management in Identify template

[tdipisa]

Description

1. The undefined text should be removed as requested.
   

2. There should be an indication in the add image tool that also a placeholder can be specified in order to get the URL from a feature attribute (maybe below the URL field). This would help a lot the user to understand how the tool can work. For example something like the one below.
   

3. Is there a way to change the icon of broken image using a better one with some explanatory text inside? For example something like the one below.
   

4. Question: is there a way using the tool to modify an inserted URL value (direct value or placeholder)? This possibility is usually present in tools like this and it should present be also there

• Not browser related

Browser info

(use this site: https://www.whatsmybrowser.org/ for non expert users)

Browser Affected	Version
Internet Explorer
Edge
Chrome
Firefox
Safari

Other useful information