Glad you can upload avatars now.
there are two areas to check for uploading,
Under Users and Groups > Manage user groups,
click on each forum access,
check to see if uploading is checked to allow.
And also edit group for the groups you wish to allow,
check to see what upload size you set,
be sure it is greater that the default 0, set it to a reasonable size.
As to using html in sig,
when you modify your sig do you see the http:// just below the bold button?
There is a great source of info at http://www.invisionboard.com/support
Hope this helps