How to create a picture, that does not loose its quality when inlarged times (x) 2 ?

Hi, I just created an effect in Photoshop, almost 700px X 1000px. But, when I made is a background for a PC with 900px to 1200px the quality dramatically changed.
So, my question is, is there any way I can make a picture that is able to expand without loosing much of its quality? I think this has to do something about Image DIP ( Just a thought, don't know what it means ) that is you make an image in photoshop with high resolution it can remain with a good quality. But, if this is true, I do not know what to do.

 

IN general you have two options. First one is go for a vector image which can be resized like you want and keep the quality or you start of with the biggest solution you will need and cromp it to the smaller size. Working with a high quality version first will enable you to still have it in a good quality after resizing.

 

You can't do much with it, just make sure that once making a bigger image from a smaller one, use "Bicubic Smoother", and use "Bicubic Sharper" from bigger to smaller.

But it's the best to make an image with the dimensions that you need.