I'm using Twenty Ten. Have uploaded header images several times over the six weeks I've been working on my blog using this theme. But today, when uploading a new header image file (.png at 940x198...the exact dimensions specified for the header for this theme) I:
->clicked browse to select the file...okay
->Firefox 3.6.13 indicated it's uploading but returns a relatively blank page with just the left hand menu bar (Dashboard, Posts, Media, etc.).
This same thing happens to me when creating blog posts and seems to occur if I've saved and previewed the file too many times. I've been able to resolve the individual post problem by copying and pasting the content of the post into a fresh post window and then deleting the original. But I don't know how to workaround the issue with the header image not uploading.
Thanks for any suggestions.