Here are the official wallpapers for Again from YUI’s official site:
The size is 1280×1024 pixels.
You can get wallpapers there by playing a slots game. If you don’t want to play you can just enter the URL of the image. It has the following format:
http://www.sonymusic.co.jp/Music/Info/yui-net/slot/wp/[the serial number of the CD]/img/[wallpaper width in pixels].jpg
You can find the serial number in her discography (eg. SRCL-7039) and available widths are 800, 1024 and 1280.
Or you can just download them here