The resolution of the images should be 300 at the size that they will be printed. So if you open them in photoshop and check the image size, it will tell you the resolution but you have to check it as compared to the image size. If it says the res is 72 but the image size is very big, that could work because if you will shrink the image in your design the res will go up…i.e. if you shrink it about 4x smaller than the size it says the resolution will go up 4x to close to 300 and that would be fine. If it says the res is 300 but the image size is very small, that might not be good because once you enlarge it the res will go down, ie if you make it twice as big in your design res will go down to 150. So you need to have some sense if the res will be around 300 or higher at the size that you will use the images. Could be deceptive on screen since the monitor is less sensitive than a printer so lower res could still look ok on screen but not when printed, and also you could be viewing it on screen at a diff size than print size so you need to check actual print size as compared to res. That is one of the main issues that affect image quality. Also format can make a difference, PNG and GIF won’t print as well. And look for other obvious quality issues such as lighting, contrast etc. which possibly can be tweaked in Photoshop but always easier and best if the original image is good.
