As described by Robert while investigating another bug report: https://bugs.launchpad.net/mahara/+bug/1219499/comments/5
The problem where some images are not showing up is because they tend to be protocol relative urls to the images and the Mahara system tries to append a url to them but this makes the path incorrect.
eg for slideshare (http://www.slideshare.net/rss/latest)
There are image links in the feed like:
//cdn.slidesharecdn.com/ss_thumbnails/lptrnhc2008cbnnhtnghththutit360-130902225125-phpapp02-thumbnail-2.jpg
and mahara sees this as a relative url and appends http://www.slideshare.net/ to the front if it which then makes
http://www.slideshare.net//cdn.slidesharecdn.com/ss_thumbnails/lptrnhc2008cbnnhtnghththutit360-130902225125-phpapp02-thumbnail-2.jpg
which is an incorrect url and the image will not be found there
Need to alter the code in htdocs/blocktype/externalfeed/lib.php from /src="(\/[^"]+)"/ to something that doesn't try and update protocol relative urls
Since every site is now virtually on https this is not relevant anymore.