Sometimes images require re-sizing. If an image is too big, it may hide other images in your email campaign and stretch the template. Check older emails, where the image looks right and the email displays correctly, then using the image icon in the toolbar check what width this image was.
Using the link below you can upload images from your computer, crop/edit them and then download a smaller web friendly version.
If your file is above 120kb, use the link below, upload the files to this site and Tiny PNG will compress the images. This means mobile users will not waste their data allowance opening your emails. https://tinypng.com/