After an update of Styles for the Twenty thirteen theme I'm no longer able to add any images to my blog (pages).
I'm not sure whether there is a correlation with this update, but I suspect it.
I tried to deactivate the plugin but that does not resolve anything.
I'm able to go to the media library, open an image and copy it (Ctrl-C and Ctrl-V) to its intended location.
At it's destination it is possible to edit the image (change the alt tag).
Does this ring a bell?