What defines "high" resolution?
"High resolution" is a relative term. Compared to a low-resolution image, high-resolution images have more pixels, lower compression, or both. These images have a higher quality, and also a larger file size.
High-resolution rasterized images, such as photographs, appear crisper and more distinct. You can see this increased clarity in our example photograph to the right in the detail comparison.
Vector graphics, on the other hand, are resolution-independent. When an image is rasterized (converted to pixels), the vector image can be scaled to high or low resolution as needed. If they are rasterized to a high-resolution image, their artifacts, such as jaggies, are less pronounced.
If "high resolution" is used alone, where "resolution" is a noun, and "high" is an adjective, it should not be hyphenated. For instance, "That image was printed at high resolution."
If "high-resolution" is used as a compound adjective, in which both words describe another noun, it should be hyphenated. For instance, "That is a high-resolution image."