Here is how you make it your signature:
1. Save the image
2. Go to "Settings"
3. Go to "Edit Signature"
4. Go to the bottom until you reach "Signature Picture" or that area.
5. Go to "Choose File"
6. It'll bring you into your files.
7. Go to where saved pictures go
8. Click the pic you want
9. Open it
10. Hit "Save"
11. Hit "Save Changes" when you're finished with your sig.
12. Voila! Your new Sig pic.