PLEASE HELP!!! + Cute Gifs!

Hello friends!

These are just some dancing images. (I don't know what to call them. Are they considered gifs? I don't know..)

PLEASE HELP ME!!

   ~ Could someone please tell me how I can have a picture from my computer be my blog background? I just can't figure it out. Do I download another blog template from somewhere on the internet? Could doing that make my computer slow and wonky? I see people's blogs with such cute backgrounds. Where is there an option to make your own picture your blog background on blogspot.com? 
THANK YOU IN ADVANCE!

That's all for now, friends!