sometimes when I come across this type of situation I add a boarder around the whole picture trying to square the picture as best I can. Then I will square out the whole block using the added boarders. Like someone mentioned you could do the Wonky type blocks if they are way off but I find most will square up fairly well without much trouble.
One other thing that helps is it to try and use a fabric that blends into the picture's background so it doesn't catch the eye as quickly. If that doesn't work I go the opposite direction and you a fabric that really makes it stand out. You will need to play with each picture to see what works best for each one. They don't all have to be done the same to work well together. Similar is good.