In case this will help anyone, here's what I finally got to work so I could edit my preferences, insert an avatar, and have at least a rudimentary signature.
- I had multiple errors that were appearing when I tried to do them, including a password error and a numeric error. Those errors were keeping me from saving any changes that I tried to make.
Edit My Preferences
Click the Private Messenger Tab
Deleted my user name that appeared in the "number of replies per conversation" field
Inserted a numeric value (I randomly picked 20 out of the hat!) into both the "number of private messages per page" and "number of replies per conversation" fields.
Back to the account tab
Deleted the automatically-inserted password that was filling the 2nd field
Back to the preferences tab
Click preferences options
Delete what was appearing of my old signature
Now, once all the gunk was out of there, I've been able to get somewhere. I was able to insert a siggy pic and some text (can't wait til that 255 character limit is lifted!), and also upload my avatar picture. (Avatars are under the account tab, and the upload process is much easier than under the Lithium system, at least now that I got it to work!