I've spent the last few days scaling HD images of popular stuff to fit the size of Xbox gamerpics, ended up with 839 of them. Feel free to take them for your
XBox 360 Background Creator by MrRockRock on DeviantArt
How to change your Xbox profile picture with a custom gamerpic | Windows Central
Create a Customized Gamerpic for Your Xbox Live Profile
gamerpic too small - Microsoft Community
What is the Best YouTube Profile Picture Size? (With Examples)