Praying in a shirt which has the picture of an animate being is disliked. This means that the prayer will be valid but void of its rewards. To remove this dislike from the salah, the image either should be so small that if it were placed on ground, then a standing person would not be able to distinguish its features or the image should be headless because an animate being cannot live without a head or the image is so distorted that no features can be distinguished.