PNG images can be up to 4096 × 4096 pixels.
Run a fullscreen dead pixel test with white, black, red, green, and blue backgrounds. Change colors manually or use automatic cycling to inspect the entire display.
Inspect the entire panel at a comfortable distance, then look more closely at suspicious spots. A defect that remains dark, bright, or the wrong color across several backgrounds may be a dead or stuck pixel.
Enter fullscreen and move through each test color slowly. Check the center, corners, and edges; defects can be easier to see on one color than another.
Clean the display first because dust can look like a dark pixel. If a mark moves when you change your viewing angle, it is more likely on the surface than inside the panel.