You can add the below CSS to your Campaign's email template.
To do so, choose the relevant Campaign and then navigate to the tab Template and Style → Settings → Custom CSS and add this code:
<code>@media (max-width: 600px) { #block_XYZ img { width: 100px !important; } }</code>
Here you can adjust to what screen size (by changing the max-width argument), to which elements (by specifying the #block_XYZ element) this should apply, and what width the image on smaller devices, e.g. phones or tablets (by adjusting the width argument) should have. You can get the element_id when you hover over a given element in the Email template preview.
An alternative solution might be to go to Template and Style → Settings → Item (mobile) and change the setup for images, but this would change the image setup for all blocks not only Image blocks but also dynamic blocks, static blocks, etc.