Poor printing and resizing.
Many images are low resolution to begin with. They might have been taken on an older camera or may have lost quality after being repeatedly downloaded and uploaded online.
De-pixelate with these tools.
Pixelation can be difficult to fix, especially if it’s heavy, but Photoshop has a suite of tools designed to improve and repair or de-speckle unsightly pixels.
The sharpen tools available in both Photoshop and Camera RAW help you manually sharpen individual pixels to create higher-resolution images.
If upsampling and sharpening don’t get you where you need to be, use effects like Gaussian blur to soften images and smooth over those rough pixels.
Use the Soft Light blending mode to alter exposure and contrast within an image, applying light and shadow to mask and conceal pixelated areas.