Images are automatically run through some optimization code when they are uploaded, which both compresses the file and potentially resizes the image to better fit into an email. This greatly decreases the overall file size of the email and images which will lower how long it takes to download and render when a recipient selects to view the email. This in turn will greatly affect deliverability and is a proven best practice for email marketing (emails that take longer to load are less likely to be read).
IF you prefer a full size, high-resolution, image you can upload the image to a location on your own hosted website and then hyperlink the scaled down image you embed in your mailing for the app. This allows anyone who receives your newsletter to click the embedded image and launch the full-screen version for better viewing.
Remember we strongly encourage you to use the .png format when available. It has the best compression to image quality of any image file type. A png should always look better -- regardless of compression and resizing -- then a comparable jpeg file (and will be a smaller file size too!).