You want to avoid using images taken from social media sites, or from email accounts, because these are usually compressed to make sharing quicker, which decreases the file size and often results in pixelation and a decrease in image quality.
Generally, we deliver our work within 24 hours. In the case of the massive volume of files, we take 48 hours but after discussions with our clients, we set this turnaround time.
Upload the highest resolution photo you have if you plan on printing your image. Notify your editor if you have a print size in mind, so they can check your image dimensions to confirm printing at that size is possible.