I upgraded to 3.8 yesterday and since then images are not uploading completely (they worked fine before the upgrade). I'm not getting an error as such - if I go to the media library and add new, the upload appears to work but (a) there is no thumbnail (b) the images won't appear in the header (or whereever I am trying to put them). Looking at the upload folder on the server the image does appear to have uploaded successfully, it's almost as though WordPress doesn't know where it is.
I have seen a few other posts about this and have tried some of the suggestions:
I've disabled all plugins
I've renamed the plugins folder
I've tried going back to the default (2013) theme
None of these make any difference.
All advice welcome, but please note I'm new-ish to WordPress.