Rasterized images or pictures should be 300dpi at the intended size to print (or at bleed size if the image is meant to print to the edge of a piece). If you are working in Photoshop with an image, go to the "Image" dropdown menu and then select "Image Size." An Image Size box will appear where you will see a couple of options. Look at the resolution setting and see if it's at 300dpi. If not, change it to 300dpi and make sure the Resample setting is UNCHECKED. After you change the image to 300dpi, you will notice that the dimensions will scale accordingly to the new dpi set. These new dimensions will give you the optimal size the image can print at.