Here is how to change the home image (or any image in the store used for design; product images can be changed/added in "Modify product" menu):
1) go to your home page:
2) right click on the center image and select "Properties"
3) write down the image url with name and dimentions:
Note: above data can be different in your store.
4) prepare new image on your computer with name "welcome.jpg" and size 537x332 pixels. Just rename and resized the image you to use there. If you do not want to resize the image, you can set new dimentions in "Edit templates" : "customer" : "main" : "welcome.tpl" template.
5) note the url of the image you want to change:
"skin1" menas this path is inside "Edit templates" menu,
next "customer_images" inside "images" folder.
In store's control panel, click "Edit templates", click "images/", click "customer_images"
6) In "Upload file to the current directory" section, click [Browse] to find and upload new file to the store. Find file on your computer, click [Open], click [Upload]. Make sure "Replace file if exists" is enabled.
7) Go to your home page and refresh it to see new image.
Let us know if you have a problem with any of above steps.