Two possibilities for receiving a "You need to be signed in" in the middle of placing a gun listing. For security purposes the GunsInternational.com site will time you out if you are not active and have not clicked somewhere on the site within 60 minutes. At the 50 minute mark you get a pop up warning. The only other option is that you may have had an internet service interruption while trying to upload your photos.
Your browser has memory and may still show your name in the upper left corner, but you may be timed out. To be sure you are logged in click your refresh button on your browser bar just before adding the information and photos in the add gun listing page. After the refresh be sure your Welcome screen name is still displayed in the upper left corner.