If you are willing to use images within your printing data or if you prefer to create your complete file in a pixel graphic program, such as Adobe Photoshop or similiar, you will be obliged to choose the resolution settings for your work.
In order to obtain optimum printing results, your pixel or image data in most cases must have a resolution of at least 300 dpi (dots per inch). Ignoring this may lead to a blurry result of the later printed product. The only exception might be given by products with big sizes which will be viewed from grater distances, such as Posters. In order to reduce the megabyte of your electronic file you may downsample these files to not less than 120dpi.
In any case, the optimum will be at 354dpi as this will be exactly the resolution in which our printing machines do work. Using a higher resolution will not improve the printed result, any resultion below that will lead to the loss of color information which will change over to blurry starting from a specific point.
Which color mode to use
By the way, colors: It is essential that for all offset printed products the CMYK color mode is used. All acceptable softwares for creating printing data will give you the option to change the color mode. If you can’t find this option in your software, you should better look for another software as it might not be thought as a software to create offset printed designs.
Use correct data from the right beginning of your design process
No matter if we speak about the resolution or the color, in all cases it is essential that the originally used data already has the right settings. In spite of professional software gives you the possibility to „convert“ all data to your whished result, this will never be done without the loss of information which may result to color changes. If you can’t avoid to use data in a color mode others than CMYK, take care that you only convert it once. Importing a CMYK image in a (wrong set up) RGB document and converting it later to CMYK again are scenarious which should be avoided.?In case of the resolution, a image data which was already downsampled once to a lower resolution can’t be „upgraded“ again later on. The neccessary image information is already lost and the only thing that you target to in doing this is representing the blurry image with a higher resolution - but still it’s blurry.