Hello,
1. Yes, images can be blocked and monitered using filetype matching. I wouldn't recommend you do this because even though they can be blocked, they can't be scanned for confidential information so you have no way to tell if the image contains confidential information (except using the filename, directory, etc.)
2. Yes, this can be done. If someone from outside the organization connects to an internal computer using RDP, Teamviewer, etc, the Endpoint agent or Network Prevent for Web will stop confidential information from being sent to the outsider's computer.
3. No, images inside documents won't be scanned. However, if a Visio diagram is embedded into a ppt or word document (using a Visio file, not a picture file like a jpeg or png) then it will be detected as a Visio file properly.
Scanning images for confidential information isn't yet a part of Symantec's DLP offering (or any other DLP offering that I know of). It would be pretty difficult and processor-intensive to scan images for confidential information as that would require optical character recognition which is not know for it's accuracy.
Hope this helps! If you have any other questions, feel free to ask.