First, make sure you've got the requisite modules for your UC installation - CCK, Imagefield, Imagecache, Thickbox (I think those are all the image related ones). Then, make sure you've told set up UC to handle images by going to Admin>Store. Look under "Images," you probably just need to click the link there to set up all of the imagecache presets and add the "image" field to your node creation screen.

I don't know for sure that you would be able to edit those nodes you've already created to include an image, you might have to recreate them.

Also, it depends on what you're using on that page to display those nodes - either Views or the default Catalog display. I'm guessing Catalog, in which case the images should automatically show up. If you made a custom View, you'll have to add the "Image" field.

Hope that answers the question.