A high-resolution JPEG is a graphics file format that compresses more data into available pixels providing an image with less loss. This JPEG format works best with photographs and detail-filled artist renderings, because it preserves more of the original work.
The size of a high-resolution JPEG depends on the file's creator. The more detailed and sharp the image, the larger the file will be, from a couple of MB to several hundred.
High-resolution JPEG files are used to store large images in a way that prevents too much leaking, or loss, of image details. The format is used by print publications, which have a minimum requirement of 300 dpi (dots per inch).
Although you can take a high-resolution JPEG image and resize it into a lower resolution, you can't take a lower resolution file and get a high-resolution JPEG image; once the extra information is lost, it can't be restored without the original image.
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Many graphics programs are available to manipulate the resolution and size of JPEGs and other images; these include Adobe Photoshop and GIMP.
While the high-resolution format is excellent for preserving photos, the large size can be a problem when emailing; some email services limit attachment size.