I only have slides and prints, does that mean I'm out of luck?

No, however it will cost you a little bit more. Many one-hour photo labs will convert your slides to digital images and put them on a CD for you. They may also be able to convert them to the resolution and size you need for your AoC listing. Once you have the images on a CD, just transfer them from the CD to your computer. If you need to change the size or resolution of the images, you will need some sort of image editing software. Once they're ready you can upload them when you create a listing for the piece of artwork in question.
Alternative: If you don't want to mess with resizing your images or uploading them from your computer, get the digital images from a quick photo lab and send the CD to us along with a clear list of how you want each image described, and what filenames on the CD correspond to what descriptions you give us. We will resize them and install them to your web portfolio for $10 per item.