I can't think of any reason for not being able to edit (including pasting) the contents of the "Edit Signiature " box. This shouldn't be something that needs cookies cleared or anything like that.
To add a graphic, you'll need to check the "Signature is in HTML format" box and paste in the URL of the graphic you wish to use. Smileys are a little easier!
I just made this my signature in a test forum I run:
Greg <smiley image="smiley_beam.gif"/>
I got the code for the graphic by copying the code for a smiley that found by going to edit a post in my forum that included a smiley and pasting that code into the signature edit box.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.