Does anyone know how to add a background picture for a header in a blog?

I can’t get it right! I have a picture that i got off the net, that i added some wording to, but when i add the link in the block that the blog design program says you must put it in for it to download it to your blog, then it does nothing, it only seems to work when i download something from my computer, and not try to add an internet link. But i need this internet link background because it’s got “animated” writing on the background, and i’d like for it to remain animated (it’s glitter text).

This blog works with CSS program too, but i have NO clue how to work that. So even if i get this background on there finally, i won’t know how to centre it, because it’s slightly to the left of the middle of the blog page too.

Can anybody help, pretty please?