When you design your creative using images you need to keep in mind the following pointers:
Supported image file types - JPEG, PNG & GIF.
Using a layered PSD rather than a flat image enables you to create better animation effects.
Only images that require transparency should be saved as PNG. All other images should be saved as JPG to reduce file size.
You can further compress the size of a PNG file using web tools such as Tiny PNG or ImageOptim.
Creatives should be 500kb or less excluding video. The smaller the size, the more likely the ad is to load quickly.