Either the JPGs, themselves, are corrupted, or the previews of the JPGs are corrupted, or the display of them is. You can determine which of these is the case by viewing the JPGs in another application, or even in Develop, and by rebuilding the Previews for them from the Library menu, and by updating your video drivers.
Also, you should just upload a screen-shot directly to the forum using the little camera icon at the top of where you type in the text on the web-version of the forum thread. You can also have others look at a JPG that seems to display strangely, to see if it is corrupted, by putting it on DropBox and replying with a public download link to it much as you did with the screen-preview.