Ways of reducing the size of an image file:

• Resize the image.
• Crop the image.
• Decrease the quality of the image (if you're using a lossy format such as .jpg).
• Switch to a lossy format (such as .jpg), especially if the image is photo-like (i.e. no hard egdes).
• Lower the number of colours saved. This is particularly useful for lossless formats such as .png.
• Lower the number of colours used. This is particularly useful for lossless formats such as .png.