Changing background image

[i_am_coocoo]

Yesterday I found a how-to post in this community with the instructions how to create a changing background image via theme layer. Today I wanted to post a request for a little clarification, on how to make the background image repeat all the way down(as after I followed those instructions, I could only get the row of images appear at the top). But for the life of me I cannot find that post. I looked through all possible tags including "header", "background", "theme layer".

In a word, I want the snowflakes on my background to appear as if they are falling down.

Help please?