Optimize the Result by Doing the Correct Things From the Right Beginning
In order to obtain best results, your images must have a resolution of at least 300 dpi (dots per inch). Ignoring this may lead to a blurry appearance of the printed product. However, for large format prints which are usually not viewed at from a very close distance (i.e posters), the resolution may be reduced below the above threshold but should never be lower than 150dpi.
Important: Please keep in mind that any layout software in which you are placing your pixel images is changing the resolution when scaling the images. By scaling up, the resolution is downsampled and scaling down will lead to a higher resolution. With that said it is best practice to draft your images with the correct final output dimension, directly within your pixel graphic software, in order to avoid any scaling later in the layout software.
Which Color Mode to Use
It’s essential to use the CMYK color mode for all offset printed products. Any professional software you’re working with for creating your design will give you the option to switch the color mode between RGB and CMYK.
Use the Correct Data From the Beginning of Your Design Process
In order to avoid unneeded conversions of the resolution or the color mode later in the design process, it is best practice to start with the correct settings from the beginning. Your first step should always be to think about what you’ll need at the end and to start with the according settings for the entire layout, as well as for all components you’ll use within your layout.
Typical scenarios which should be avoided are, for example, to import a CMYK image into another RGB file in order to convert the entire set at the end back to CMYK again. Or downsampling the resolution of an image first and scaling it up again later. All kinds of conversions will cause a certain loss of information which can’t be recovered again.