Pixels per inch, or ppi, is simple a way of converting pixels to inches. You can find out the dimensions of an image in Windows either by hovering over it and waiting for an info box, or right click > Properties > Details tab. Sometimes the image will have a ppi already set, but it’s easy for us the modify it.
For optimal use in print, we must receive images that are 300ppi or higher. To figure out how an image will print, divide the dimensions by 300. That means that a 300px x 300px image will print at 1″ x 1″.
Below is a 300px x 300px image and an example of how large it will print on letter (8.5″ x 11″) paper.
Resolutions on computer monitors are usually much lower than 300ppi, so images posted on the internet are almost never suitable for print. As another example, if we wanted to use an image that spans the whole width of a page, 8.5″ x 300 = 2,550px, so the image would have to be at least 2,550 pixels wide. That’s more than double the width of many computer monitors.
In addition, .jpg, .tif, .png, or .gif are not acceptable formats for logos. If you pull a logo from a company’s website it will almost always be in one of these formats and cannot be enlarged to any useful size without degrading. These are pixel-based formats and logos should be optimized in a vector format such as .ai or .eps. Vectors are infinitely scalable with no quality loss; if it’s hard to imagine, think about fonts and how they are only based on curves, so they can be enlarged to any size. I always recommend asking the company directly for a vector version of its logo, or you can try to find one in a vector database such as Brands of the World.