Depending on file type and adjustments made, you may notice that Delivra's image editor (powered by Aviary) changes your image resolution during resizing.
WHY DOES THIS HAPPEN?
Aviary, the 3rd party image editing app in Delivra, retains the original image format (.JPG, .PNG, .GIF, etc) when saving over images. Some image formats are better suited for image compression than others.
In the example below you'll see the .GIF format is especially poor compared to .PNG format due to what it can do with transparent pixels.
Aviary is a simply photo editing program, made to be quick and easy to use, and does not have the settings options of a more powerful editing software like Photoshop.
The recommended image size in Delivra is already ~800 pixels wide or less. This is a mid-range size so the image displays well enough on mobile devices with retina screens (where you want the image to be much larger than the screen to display sharply) yet is small enough to reliably be of a small enough file size to not inhibit download speeds. It's best to upload ~800px images and let them automatically re-size in the content block in the Delivra email layout so that they will display sharply everywhere.