There are many threads with the same topic, you could just have checked them for help but well
You could use imgur.com like I do. Open imgur.com, you don't have to register just drag on drop your picture on the side after you opened it. After it uploaded move with your mouse over the picture, a little arrow should appear in the right upper corner. Click it and then click 'Share files' (I hope that's what they called it). Copy the BBC Forum Code and past it when you wanna create a new profile picture or signature picture.