> How do i resize pictures so they appear smaller and
You have to resize images off-line before using them as thumbnail.
You can use a free application called "irfanview" from http://www.irfanview.com/. It allows you to re-size hundreds of images at once. Let us know if you need help with this.
> how do I get a larger picture to appear in the detail box.
Go to "Modify product" -> select product -> "Modify" -> scroll down to "Detailed images" -> "Browse" -> Select the large image on your computer -> click "Upload" (you might need to scroll down to this section again to click "Upload" button.