Unfortunately, if your image looks pixelated on the screen at 100%, then it is likely it will print that way too. 

We recommend that you check your artwork thoroughly before you send the files our way for book printing. Images of more than 300 dpi is recommended!

It's important to make sure that your artwork is designed to suit the size of your print. For example, If you are uploading A5 artwork, but you want to print your book in A4, then the images will become distorted.