Is any way to prevent from decrease of image quality that we painted in background of a Jpanel when we resize the frame?
If you resize an image of let's say 100px in width to 80 pixels, then about 20 pixels need to be cut off. There are several solutions for this, but all of them have their drawbacks. In your case I assume those missing pixels are "averaged", which will cause the image to look still correct, but somewhat blurry.
If you want sharp images, you need to either prepare an image for each size, or make sure that resizing only happens in a power of two (so half size or double size), as then the result looks noticeably sharper.