Blurring of images usually happens under these circunstances:

1. When you are working on a page or object that is bigger than 2400px width or height.

2. When working with selections that are bigger than 2400px width or height.

3. Text will become blurry if you convert it to raster image and scale it up.

4. Raster image will become blurry if you scale it up.

So, to avoid blurry images make sure that none of the above is happening.