Need help resizing picture to poster

So i have done quit alot of research and can't really find an exact answer. i currently have this image


I want to put this picture on a 20x30 paper. The thing is that, i tried many ways of preserving the quality of this picture while resizing it to a bigger picture. It seem like that everything i tried, it still looks the same. Can someone please help me out? it doesn't have to be a perfect but try to preserve as much of the picture quality as possible.

You cant make a bitmap image bigger without it looking like crap.

Problem is here that when sizing a picture, going from big to smal the result will be good as the pixel will keep a ratio size/RGB that is proportional, when going from small to big then the ratio is non proportional and the size is bigger than the RGB info and the quality and spacing of the colours are gone, ditto crap picture.:o