Look at the html email source (or webversion source) and copy&paste each image link url into a browser to see if it shows up. If it doesn't show up, the image link is obviously wrong.
If you've uploaded the images to a ESP like Mailchimp for example, I'd follow up with them as to why the images are not showing. Typically they will create a url for the image, so they have control over why it isn't showing.