Remarkety templates are designed to produce responsive emails. A responsive email is one that is designed to look good across multiple devices.
In a standard Remarkety template, your logo size is 300px and the banner size is 600px.
If the image you've uploaded differs in size, the images may appear pixelated or stretched. You will need to re-upload the image with the correct size format in order for it to appear clearly across multiple devices.
If you need additional assistance with sizing you image please contact us at firstname.lastname@example.org.