The short answer to this question: the dimensions to use are completely up to you.
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That said, we've had many ask for a recommendation or guide to the dimensions that should be used for the logos or banners. If in doubt, you can't go far wrong with the following dimensions:
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​ Logo: 250x250 pixels
​ Banners: 1024x500 pixels
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With the above, the width is mentioned first. With either, you might not want to go much bigger, as you are otherwise pushing your products further down the page. The logo appears on every page and often sits at the top left corner, where as banners are there to showcase your work (a little like a billboard or poster), will likely only appear on one page and thus justify the larger size.
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A great way to get an immediate recommendation is to preview your theme with the Demo Content. You can do this by visiting the Design section of your SupaDupa store backend. From here, click the 'Demo Content' button at the top right and on the left, the preview will include a logo and banner which can serve as the recommendation for that theme.
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As for resizing your images, this is something you'll need to do before you upload the image.
That said, the banners are automatically restricted to the width of the page when loading.
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