This can be achieved through a Custom Profile Field added through the AdminCP and a template edit to MEMBERINFO or postbit(_legacy) if you'd like it displayed in the user's posts.
The basic code behind displaying it would be something like this (For MEMBERINFO):
Code:
<if condition="$userinfo[fieldX]">
<a href="http://myspace.com/$userinfo[fieldX]"><img src="/path/to/image.gif" /></a>
</if>
Where X is the Field ID assigned to your new Profile Field.