Creating a high-resolution image from a low-resolution original is no substitute for starting with high-rez data… no matter how many times you’ve seen it done with spy-satellite photos in the movies.
Changing the level of detail in a raster image is known as “resampling”. Going from a high-rez to a low-rez image is called “downsampling” — and while it results in a file that takes up less space, the discarded information is usually lost permanently.

“Upsampling” the resulting low-rez file (as we’ve done in the example below) almost never restores the original level of detail. At each stage, the computer is basically rounding the pixel data up and down to determine the new average values — and this tends to result in blurrier images each time a resampling process is performed.

A good rule of thumb when building web-to-print artwork is to use (or at least keep available) the highest-resolution versions you have of your source files, and not downsample the images until you must provide the final production file, when you have information about the size of your finished print.

Upsampling for placeholders
Image placeholders are checked for resolution in terms of how many pixels they contain and may not necessary need to be the highest quality images. This is one place where using upsampling is a good idea. Read about it in Image Quality and Template File Size articles.

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