This happened to me as well, except all images from my themes don’t show up, not even in the default theme? Anybody have any idea why this is?
I upgraded to 2.3.1 on two of my sites: http://www.brianlaesch.com and http://www.youtuberoast.com . Now the header images and all other images used in the themes won’t show, but they are still in the correct file directories…
It took a lot of research, but I finally found a widget plugin, Daiko’s Text Widget, that will let me insert an image in my sidebar. Check it out live at my blog, Jimboland Jots, at http://www.jimboland.com, at the top of the left sidebar.
You’ll need to download and install his plugin, activate it, and then create an image link with an absolute path to the image you wish to display.
