Ok, since we start from there lets take it all from the beginning .
When I still had the wp multisite at 3.4.2 version I didn't had this issue. The site was working without any "http error" when I was uploading a photo through media or post/page.
After the update to 3.5 version it started giving http errors. I tried with with all the plugins deactivated and having activated only my premium theme. It kept having http errors, so I decided later to deactivate the premium theme and activate Twenty Twelve theme. It didn't have any errors while I was uploading photos while all the plugins where deactivated. I decided after that to activate one by one the plugins and upload a photo everytime, when I activated Co-Authors Plus (Version 3.0.3) (while still having Twenty Twelve theme on) it showed an http error when I was uploading a photo. I started uploading many photo sizes, at first I thought that it uploads without any http error those that were smaller in size.
So, I compressed to a smaller size photos with big dimensions and it kept having http error. So I decided to upload photos with different dimensions and I saw the problem is with the dispossession of images.. specific any photo over than 511(width) i got http error , and any image uploaded less than 511(width) didn't had any http error.
So with simple logic thinking, the premium theme and the plugin have incompatibility with wordpress 3.5 with the photo dimensions when you are uploading.