Using ftp program if you do.
1. Go to your /phpBB directory/images/avatars/gallery folder, create a subfolder called "angels"
2. Upload all the images into that subfolder, "angels".
You are done! Go to your profile page, click on Avatar Show Gallery button, you should be taken to the gallery where all the avatars are. Choose your angel avatar!
I hope you are using a ftp program. If you need more further assistance, let just hollar! Don't be shy. We will help.