Restrict parser to only add images from Doxygen XML with an HTML type

[KeithBallard]

PS: sorry for the second user name. I didn't realize which account I used to open the issue.

This pull request is in reference to: #51

The open questions to me are:

1. Is my assumption about image tags applicable to all users?
2. Or should there be a configuration option to select a particular tag?
3. More broadly, should be apply to more than just images?