Your filename should:
be descriptive
contain no spaces
use hyphens (not underscores) to separate the words and make the filename as easy to read as possible
Example: An image of a girl laughing should be saved as 'girl-laughing.jpg'
Do not use a generic file name like IMG_001.jpg.
If the image's filename contains spaces (e.g. 'girl laughing.jpg') you must replace the spaces with hyphens).