This tutorial will tell you how to make HD textures from seamless texture images.

1) Find an image to use or make your own (Find one that has a higher resolution than what you need for your pack!)
2)Open the image in Paint.NET or another image editor
3)Re-size the image to the resolution of your texture or resource pack
4)Export the image as a PNG
5) Add the image to your texture or resource pack

*Note: This tutorial will only work for 1.5 and later, because it creates individual images for each texture, and will not work if you are trying to create a terrain image.