(More about image formating) Certain images (practically anything but photos) look and work better as PNG. These generally include flags, graphics, icons, diagrams and certain screenshots (such as from old video or arcade games.)
Example: -- What to do: remake the image or retouch it (making it sharp) using image editing software. More information can be found at Wikipedia:How to reduce colors for saving a JPEG as PNG. Note: you should not just convert JPEG files into PNG files without doing anything else, since this won't improve quality and will just make a larger file.
PNG is not an efficient format for photographs. These should be converted to JPEG to save space.
What to do: save the PNG on your computer, open it with an image editing program and save or export the file in JPEG format. Try saving with different quality settings and choose the lowest quality (highest compression) that maintains the appearance of the image.
Pictures that need to be retouched, trimmed and scaled
These include images with captions that are too small to read or photos that contain irrelevant surroundings. Dark and unclear pictures should also be properly modified. Background color of graphics and diagrams should be white or transparent.
Some pictures just aren't pretty looking, and new versions with higher quality should be found and used instead.
What to do: make or look for a substitute. Be very careful if you use the image search feature in most search engines; the vast majority of pictures on the Internet will be unsuitable for the Commons because of copyright problems.