Hi Gary
If you have access to Photoshop, you can achieve it following this method:
1. Open the image in Photoshop
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2. In the tools palette, select the ‘Rounded Rectangle tool’:
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3. Now mark out the area of the image you want to appear as round-cornered:
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4. In the ‘Paths’ palette, select “Make Selection”:
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5. Then in the ‘layers’ palette, make sure you click on ‘Background’ layer (ie the image):
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6. Then go to the ‘Select’ menu and choose ‘Inverse’. You can then simply hit ‘clear’ providing you have white chosen as the background colour:
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That’s it, job done, just crop to finished size, save and upload!
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Hope that helps, good luck with your site.
Chris