pixels problem

I have this little annoying problem that started all of a sudden and I hope someone can help me: whenever I try to cut an image so that it's 100px X 100px the pixels get messed up, like this:

Before... ...and after


Here's another example: before... ...and after


As you can see the pictures I use aren't blurry or bad quality photos, so I wonder what's the problem... anyone can help me?

Thanks in advance to everybody!!! :)

Edit: already solved, thank you so much aquamarcia =)