If you create a Digital image on a device that records it at 72 DPI, and try to convert to a high resolution say 180 dpi, it will “shrink” the image to about 40 percent of the original view. If you want to increase the size along with the resolution then your pixels will double in size but the image will be reduced by more than half, which often causes the “:jaggies” saw toothed pixel edges.
All of this is based on mathematical models. You have 72 (dots)pixels per inch in a one inch image. If you want to convert, you must stretch or minimize each pixel in the same space. If you want to resize (enlarge) the same precedence will occur with the addition that now the pixels will also be stretched. As you can see a problem occurs when you want to convert a low resolution image to a high resolution image.
You can take a high resolution and downsize, or downsample and it will look good, but when you try to enlarge a low resolution image it will stretch the pixels and make the image look very bad, unless you compensate by reducing the size. Catch 22, so you always want to create digital images that are of a high enough quality they will not have to be enlarged.
There is software that can smooth, anti-alias low resolution pixels into high resolution images, but the software programs come with a price. Check out such a program at www.imagener.com
Got to here to learn about resolution: http://www.ltlimagery.com/resolution.html
If an Image was saved in 360 dpi, then it has 360 pixels per inch, saved in 180 dpi then 180 dpi per inch, etc