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If you want to make a circular image (which has an invisible square outline - unavoidable), then you can use the latest ImageConvertor to convert your image - there are other extensions...
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If you just want to 'display' the image as a circle, use the button component, set to square dimensions, and Oval shape.
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In a webviewer, you would be doing similar things using html/javascript/css on an img or canvas tag.