I'm sorry for the confusion! The maximum image width supported on Behance is 600px wide. Occasionally you may see projects that seem to have wider images. This is a "hack" some members have created by uploading a background image that seems to blend in with the edges of the 600px wide content.
You're welcome to do the same on your own projects, but this is not a supported usage of the background images and we can't guarantee that future changes to our project editor won't cause a conflict with this unorthodox use case. Right now, these projects are not supported on mobile devices because of iOS background limits, so we don't recommend using background images this way.
I hope this helps clarify.