It will always fall back on the coding of the email template.
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What pixel is the With of your email?
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Is the picture original size from the source? or try to resize it with HTML
I will suggest you use Litmus as Steven refers and read post with your HTML designer so you test in larger client.
Mailchimp have a small post here.
http://kb.mailchimp.com/article/how-to-code-html-emails
You email you code shall be look good in responsive mobile clients.
This you need to create a bunch of extra CSS and thinking of this in the Table. Then your more Optimized for Android and others.
The Email shall have the same look and field on Desktop. If your Audience use
Then this is where you focus shall be. If others then this need to be include in the testing process.