You will not alter them or add elements to it.
Resizing with constrain propotions is totally fine. You are not allowed to re-share or upload them for further sharing.
Personal use for the avatars means you are welcome to use them online on social pages, boards or similar. It would be great if you add a backlink to lhiondaig.net somehwere near the avatar but its not quite necessary.
Personal use for wallpapers means that you can set them on your pc desktop, smartphone or tablet background. You are NOT allowed to upload or use them online!
If you are unsure on the term personal use, you are welcome to drop me a message so I can help you out =)