What are guidelines for images?

• The bit depth can be 72dpi. The maximum width of a full-size image is 640px, so full size should be 640px wide or double up, 1280px for full power in retina displays. The weight can never exceed 10mb because the total weight of an email should never exceed 10mb (some email clients will not let that email through).
  A normal full size image is normally about 200k.
• The file formats for images are .jpg, .gif or .png.
• Animated GIFs are okay, but certain email clients will display a static image.

Image Optimizer

Now, when uploading images into Lime Marketing to include in email content, you have the option of using the built-in Image Optimizer to automatically adjust the size and format of the image to work well in the email. This feature is automatically activated for new images. To verify that you're optimizing pictures, you will see the checkbox "Optimize image upon new upload" has been filled in when adding a new image. 

The image will retain great quality. Animated and transparent images are not adjusted.