My Portfolio Questions

Not sure where to post this questions since I couldn't find a "My Portfolio" section.

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I have a seperate image "gif" 290px width - that looks fine on every device except phone. There it blows up to cover the entire screen. I have set the image max width to be 290px. So it shoudln't in theory become that large.

Is there a way to set max width on mobile phones? Or any other way I might have missed