You may have hear that images with text should be saved as PNG but if the image has any photographic elements we recommend JPG. PNG is great for lineart. File size is just a limitation of PNG files when the images are photographic instead of lineart. The same image converted to JPEG and uploaded to Drupal compresses to almost 1/8th the size.
Example Cordova Quartet Image
Original PNG Image: 506K
JPEG Version @ 100%: 374K
Drupal resized & compressed PNG: 279k
Drupal resized & compressed JPEG: 40k
Drupal resized & compressed PNG (279k):
Drupal resized & compressed JPEG (40k):
