Any blogging platform that is supported by Windows Live Writer
that you can set up and configure for her (Wordpress (self hosted), Wordpress.com, Blogger, etc.)
It a lot easier than dealing with built in web based post editors and especially fantastic for inserting images.
Lot of options for resizing, borders, positioning, linking, and whether it links to full size or something else, and whether it opens in a new window, watermarking, etc. You can even set up defaults for her so it does exactly what she wants when she drops an image into a post. On the first post, just set everything on an image and click to make those setting the default.
It feels just like typing right onto your blog page (theme aware), And it supports drag & drop of images, right from Explorer. And when she is finished and has the page just how she wants it and clicks the publish button, it can even be configured to open the page she just published in the default browser so she can check it (and edit more in WLW and republish it, if necessary)
The only thing it really isn't good for is attaching files for download to posts (zip, exe, pdf, etc.) For that kind of stuff, I recommend dropbox, which one can drop the files into a folder and then right click the file to get the URL to link to, for visitors to download it.
It really is the best desktop blog editor that exists, though...so easy to use, and amazing that it's free. It's also the best way to prevent beginners and non-techies from getting themselves into trouble.
If you decide to go with self hosted wordpress and need some help setting it up and configuring it, let me know and I'll help you. (I am in the process of getting tomos set up with a wordpress powered portfolio site on free hosting, free domain name, etc., and it would be trivial for me to do the same for you, too)