How can I convert a jpg file image to a high resolution image?
High resolution just means that there is huge amount of pixels on the X and Y axis. High resolution doesn't equate hight quality yet both terms are closely related because of misuse. High quality depends on the amount of pixels on a square inch/centimeter/milimeter/etc. Usually high resolution pictures have a high quality because if they didn't you would end up with a pixelated pictures.
As Mike said if you stretch a picture in any direction and the picture is not high quality you will end up with a pixelated picture.
What do you mean by high resolution?
In general jpeg is a compressed file format which means if it's quality is insufficient there is only (very) little you can do to improve it. If you want to increase the resolution (e.g. double the width and/or height) the quality will deteriorate even further.