How to improve image quality in photoshop ?

Select your image. Try to find a standard-quality picture, that's not either very small or very big.

Duplicate your picture by Layer>Duplicate Layer.

Right click on the layer you have just created and select Blending Options.

Set it to screen and select low opacity. Then select Layer>Flatten Image.

Duplicate your picture (Ctrl+J), and go to Filter>Noise>Reduce Noise. Now the settings depend on your picture, so experiment to get the best results.Here are the settings which are used in the picture above:

• Strength: 10
• Preserve Details: 7%
• Reduce Color Noise: 0%
• Sharpen Details: 56%

Now that you've improved the skin detail, you might still have scan lines on the background and hair, so go to Filter>Blur>Surface Blur. Once again, these settings depend on your image so just experiment.These are some sample settings:

• Radius: 2
• Threshold: 8
• The example picture after that change looks like this.