Well, the easiest way is perhaps to just copy an avatar with frame
to your harddisk. Open it with gimp. Paint everything black/ cut out
everything that's not the frame and voila you have an empty frame
that's 96*142. Now measure the inside, it's ca 86*132, and take that
as a reference for your selection tool. Make a fixed selection on any pic
you like and copy the selection into your empty frame. It should fit nicely.

I've messed it up a little for Uesugi Kenshin's avatar. That is, one pixel column
on the left is still black.