I had time ago the same question. Because I think it's a serious usability error not be able to preview the images you're about to attach (in any webmail happens as well, in fact, in any 'upload image' in the Internet), or even, you're going to open. I asked some engineers I know about this problem. They told me that was a problem of the GTK libraries, because you can preview images when attaching in KDE (with QT libraries) for example. And that's why also you cannot preview them in XFCE, because it use GTK as well. I think it's a serious issue to deal with. Some programs, like GIMP, for example, add a panel to show you the preview, by I think it's more like a 'trick'. It would be nonsense to be searching and image in GIMP, but not be able to preview it! So the problem is not the Thunderbird version, instead the GTK libraries.