How to get resized image for battlenet without losing qualit

I have seen on battlenet that there are images in 225x150 pp scale with high quality resolution (despite the fact it is just 225x150).

I resized my image and it becomes blured, can anyone help me with that? How can I resized and image and save it as TGA without lose resolution scale when importing it and setting it on my map using battlenet info image map for an arcade?

The way I do it is by using a free program called GIMP. You can use gimp to shrink the image to the much smaller size, and you can choose between different scaling methods to get whichever one looks the best. Then I just save it as a .dds file (a file format understandable by SC2) and import it into the map.