[noypiscripter]

For adding signature

1. Click your username at the upper right corner then choose User Settings.
2. Click Account tab
3. Check "Show Signatures" checkbox.
4. Click Edit Post Signature. A dialog with editor will show up where you type your signature.
5. Click Save
6. Click Save Changes.

For the other one, since you added the banners in the forum description, they will appear in the forum directory on homepage and in each forum channel page itself. A workaround is to hide the banners in the homepage via CSS. Add this in css_additional template in AdminCP.

.forum-desc img {
    display: none;
}