Submitted images should be free of noise, dust and artifacts. Crop neatly and with space for words if desired by purchaser. Colors should be accurate and not oversaturated. Use the highest resolution your camera can produce. Check with an agency you are considering submitting to for more specific image parameters like color space, compression, or file size. Once accepted you will need to continually submit new images, check with the agency to see if they have specific image needs or what types of images are big sellers.