Replacing placeholder images
When replacing placeholder images, it's important to be careful that you don't erase the content table that they are placed in. That would remove the spacing/padding that may be associated with the template, and will remove the responsive coding the resizes the image for mobile viewing.
All images should be 600px wide and 72dpi. The height can vary,
but we recommend that they be no taller than 300px.
1) To replace an image (or placeholder image in Saved Content), right click on the image and click properties. If you are replacing a grey box placeholder image, you may receive an error message that you don't have access, you can just close that error message.
2) Click on the image icon next to the "Choose Image" and then select your image as normal.
3) Do not forget to enter the alt text into the box! Then click OK to load it into your email.
4) Before saving, double check that the table outline (red arrow below points to it below) still appears around your hero graphic when you have it clicked.
If there's no table, you'll have to undo your image placement, and try again. That surrounding table is important to keeping your email properly formatted.