Does this work for Mailerlite

Yes it should work for any service that uses RSS feeds.

Blurry images usually means the size selected in the plugin’s option page is too small… have you tried increasing it to Large, or using “Full”, which is the original full size unaltered? If still blurry using Full, then the original image size uploaded is too small, upload a larger image. Also, be sure you have the “Resize images” checkbox unchecked on the RSS feed page (in Mailchimp, and maybe other services as well.)