Some ways to eliminate or reduce pixilation:
1) Use a higher resolution image file.
2) Incorporate an image file less prone to pixilation in the vector category instead of rasterized.
3) Build the game to handle compressed images, then start with increased size in the creation phase and compress to the correct size for import into the game.
4) See if game performance would let you use lossless images such as BMP.
5) Create procedural images with shaders.
I wrote this to maybe cause you to reconsider your image implementation. Are you ready to grow?