Find it under "BlogSite Options->Blog Page Options"
The blog pages can have a banner image across the top. It can be the same image on every page, a slideshow of up to five images, or you can have the slideshow play on the main blog page but have every other page in the blog section just use the single image.
You can add up to 5 images for your blog page slide show. Sizes should be 1000 pixels wide and 150-280 pixels high. (CuppaJoe Blog Only version must be 185px).
Note: Make all your images the same height. Different sizes will cause the slideshow to display incorrectly. ALWAYS remember to save for the web at 72 dpi to reduce load time.