It is pretty much guaranteed you have a dynamic IP address. You have to pay extra each month for a static one, and one of the reasons to have one is hosting a web site where your server address has to remain constant so others can find you.
I do not think sharing from your computer to the forum is an option for an avatar. You can share an image for an individual post, like uploading an attachment to place a screenshot.
Dropbox is an application, you can find it in Package Manager/Synaptic, which synchronizes a folder with it's sub-folders with an online, 'cloud,' storage site. It will allow you sync files between several computers if you set up the same Dropbox account on each. It has a Public folder from which you can share links to files so others can see that file. It uses 256 bit encryption. The base 2Gb account is no cost.
Compaq Presario CQ62 Dual-Boot: Mint 13 Gnome x32 PAE, LMDEREADING: The Autobiography of Benvenuto Cellini