While Klaviyo allows you to add up to 5MB per image, 1MB should work fine and is best practice. You want to think about load time, especially for mobile recipients who are not on WiFi or those without a fast internet connection.
PNG files will typically render the best, but we also accept JPEG, JPG, and GIF file types.
For image resolution, we recommend 72dpi as that's the standard for web.
For the image size, the height or width should be no more than 2000px. Anything larger could also potentially cause load time issues. If your image is more than 2000px in height, slice it into multiple sections so that it takes less time to load.