Eep! Help!

I don't know if this has already been asked/answered, but I couldn't find anything. If there is, direct me to it please, i'm sorry for taking up space and time.

Whenever I resize a picture to 100x100, it turns out weird.
See the kind of messed up lines?? How do I get rid of that? o_o;

Thank you in advance!
<3
Liana