How do i simply change a sprite's image in cocos2d?

I've tried

[[CCTextureCache sharedTextureCache] addImage: @"still.png"];

But I always end up with a distorted image for some reason. It's most likely because my images are not the same resolution, but for this app, they can't have the same res. How do I change the sprite's image without going through the complicated process of making a spritesheet or an animation or any of that.