There is a way to move your user folder to another volume, but there are potential dangers and problems with doing that. For example, if something were to happen to that external hard drive, it would render your computer unusable unless you have other users whose folders are not on that hard drive.
I don't advise doing this, especially not if the only reason you have for doing so is to get a special icon on the Pictures folder on an external drive! Just store your data on the external drive and don't worry about whether it's in an "official" user folder or not.
However, if you're dead-set on doing this, here's how: