Please post the exact wording of the error - copy & paste if you can, or just type it exactly word for word....it's really hard to diagnose an error without knowing what it says, and "http error" is too generic - it could be many things.
I've been using WP for 10 years, upload images frequently, and have not had this type of error, so I'm sure something is causing it, but not 100% convinced (yet) that it's WP.....knowing the exact error will help us to help you.
Also, please forgive me if this sounds like an elementary question, just troubleshooting 101 here, but how large (filesize & dimensions) are the images you're uploading?