Since updating ot WordPress 4.2 (WP 4.2.1 now), I have had some problems updloading images.
I did some plugin troubleshooting and determined that this was the case. After more troubleshooting it came down to a conflict between WordPress SEO and Post Thumbnail Editor.
Here is a link to the thread on the WordPress Support forums
A temporary workaround is simple enough to get media and image uploads working from the page or post editor.
Just comment out line 171 in post-thumbnail-editor.php and the upload problem will go away.
/** For the "Edit Image" stuff **/
/* Hook into the Edit Image page */