ECMS-4506: [IE8] Image cannot be selected in Content Selector

Problem analysis
	- The url isn't decoded so that the js error is raised by debugger in IE although this url can be processed by server
	- There are a loop of all elements to find the url field. It causes performance degradation in IE.
	- In case Image is cached, Content Selector cannot work

Fix description:
	- Decode url before processing
	- Avoid traversing all elements of dialog by finding exactly this element
	- Only changing parameter if image is loaded (from cache and/or loading process)