If you can’t view a JPEG embedded in a webpage when you’re using Internet Explorer, the problem could well be that you’re trying to look at a CMYK jpeg. You need to resave it as an RGB JPEG. This seems like a massive problem to me, but apparently not to the IE developers as it is in the very latest version and has been around for years. I would imagine that a lot of people who have non-techy users using a CMS which they have to manage get a bit irritated by this.
The thing people seem to do is to make an RGB version from the CMYK. This might cause some strange colour issues but I suppose it’s better than asking people to try and remember not to upload CMYK (especially given that half of them probably don’t even know what it means). There doesn’t seem to be any kind of premade option in TinyMCE which actually lets you do this so, so it’s a case of coding it in yourself I suppose.