One problem I come across is people who send through images that are web quality, around 72 dpi and expect the image to be good enough for a printed publication. Often people think just because the images looks fine on their screen it will print well in a publication. As a rule most images for printed media should be at least 300 dpi. Resolution refers to the number of dots per inch (dpi), or the amount of detail the image has. Higher resolution means a more detailed image and also larger file size. You can check this by opening the images in Photoshop and checking the resolution in the image size menu. Also, as a rough guide you can click view actual pixels, to check the quality. If it appears blurred or grainy or very small, then the file is poor quality and this is how it will print.