Resolution is not a straightforward topic.

Image Resolution is quantified by Pixels Per Inch (or PPI). It is – literally – how many pixels (individual squares of a particular color) fit into an inch. As it is a direct correlation of pixels to inch, that number changes depending on what is done with the image in a layout.
What is important is the EFFECTIVE  resolution.

Example: An image shows as 4 x 5 inches at 350 ppi in Photoshop. is scaled to 8 x 10 inches (200% of its original size) in the layout, its effective resolution  becomes 175 ppi. (Double the size, becomes half the resolution)


Avoid pulling pictures and graphics from the internet These images are often low resolution, typically 72 to 96 ppi. As such, they will appear pixilated and blocky when printed. This is especially important for hard-edge elements, such as logos.

Photographic Images

Without going too deeply into why*, Photographic image resolution should be at least 350 ppi at final size.
Note ‘Final Size’ is the size of the image will be once it is printed.

(*If you’re interested in a bit of the ‘why’, see “The Math and History Lesson” below)

If there are hard edges or high contrast in an image (such as architecture, reproducing a painting, or a face with a lot of character), higher resolution will preserve more of those details. (Conversely, lower resolution risks losing those details)


Other Graphical Elements

Logos and other hard-edge elements should be vector art (such as drawn in Adobe Illustrator). Vector graphics do not have resolution, because the shapes are defined mathematically.

If you have no option but to use raster images for logos, they should be the highest resolution you have. What’s more, when exporting the layout as PDF, you should turn off any downsampling in the “Compression” section of the PDF export dialog.



The math and history lesson:
To reproduce the full range of colors/grays in an image, the resolution must be at least twice the halftone frequency (or line screen) of the output device. (Quantified by lines per inch – or LPI.)

PPI = LPI x 2

In the days of ‘burning’ printing plates using negative film and a vacuum light table, 133 LPI was typical. (120 LPI was common, and as low as 85 LPI for printing on uncoated paper.)
In those days, 300 ppi images were more than sufficient. Those days are long gone, but old habits are hard to break.