Hi, I had issue with uploading photos after 3.5.1 upgrade. I had aprox 100 files on my PC, already resized to 900x600px. I wanted to create Gallery in a post, and every time (I tried manytimes) some of them failed to upload with "HTTP Error". Different images everytime, not the same ones.
It failed on /wp-admin/async-upload.php script, which was unable to return anything in 60 seconds and webserver returned error 504 Gateway timeout (as I saw in "Network" activity) - it appeared as a red HTTP error in Media window. An original image was uploaded, but there were no thumbnails. What? Webserver needs more than 60 second to resize image from 900px to 150px???
I have googled countless times ... there are no memory issues, no not-writeable upload dir, no mod_security on etc. Some images are proccesed, some aren't.
And solution? Imagick PHP library. Script freezes on line "$this->image = new Imagick( $this->file );" in wp-includes/class-wp-image-editor-imagick.php. My webhost has ImageMagick 6.6.8-5 2011-05-03 Q16 version, quite obsolete. Now I am waiting for an upgrade. PHP version is 5.3.6.
I have forced WP to choose "old" GD library for dealing with images and for the first time I have uploaded all 107 images at once without any error.
So, I believe irregular HTTP errors when uploading photos can be related to ImageMagick library.