The ideal image resolution of your artwork should be 300ppi (referred to as dpi when printed) at the size it is intended for print.
Note that if your files are flagged by our prepress review team for low resolution, we cannot confirm that the images will print "fine." Our prepress team analyzes images for the mechanical resolution. The printed image quality of low resolution images is often subjective.
As advised in this video, you should order a single book proof if you have any doubts about your printed image quality.