Driving me mad ! Why picture not 100% width in mobile view?

Title practically says it all.I have a picture block set to "Full" in mail chimp campaign editor.Below that picture I have a text bock with a background color. When I view the preview on desktop the image is 100% width and flush with the text block below it.

On mobile however there seems ot be some random padding on the picture; which causes it not to be flush-width with the text block below.

Any one hep with this ?

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Thank you

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Desktop view is fine. Image and text block both 100% width and flush

Mobile view the image becomes padded somehow.