This is a feature of responsive templates. They display images in a smaller size, so recipients don't have to scroll to the right anymore.
You can solve this as follows:
Add a text element (look under: 1st row of standard elements, right icon).
Duplicate it to the right via the orange frame.
Remove the text from the left or right text field and add the image.
This allows the email client to display the image larger, as it can set the image above or below the text..