While using images for website or if you are designing brochures / catalogs, using round corner images instead of straight corner, gives an elegant look. To change your straight edge image in to round corners image, use below method in adobe photoshop.
First open the picture which you want to turn into rounded corners. The picture will be on it’s own layer, and then you will have to create another layer. On new layer, draw a rectangle, with the rounded rectangle tool. It simply doesn’t matter what color you fill in it now. Center the rounded rectangle. Whatever is inside this rectangle will be included in the picture. Now come back to Layers palette, select the layer which you draw the rounded rectangle on. Then double click on that layer. The Layer Style palette will open up. Then go to advanced Blending area, slide the Fill Opacity percentage as 0%.
Now go back to the Layers palette, click on the Paths tab. With current Path selected, click on the little arrow button, in the upper right corner of the Layers palette, and select "Make Selection..." from the select list. Go to "Make Selection" option box, and change Feather Radius to 0 pixels, and Anti–aliased IS should be checked. Click OK. Your new rounded corner image is ready.