You can either keep the width ie 960 OR the new hight, you can't have both
unless you except the distorsions or live with images borders however a
small vertical increse in size is usualy unnoticable.
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f=Ma well, nearly...
16:9 is defined by pixel aspect ratio. In PAL the picture size is
720x576 but the pixel aspect ratio is 1,422. So, you have to do a simple
math. Picture width in 16x9 square pixels is 1024 (720 x 1,4222). So
what you do is: You resize your image to 1024 width with constraint
proportions. It´ll give you 1024 x 768. Then you´ll have to crop the
image in height to 576. Now you have 1024 x 576. You now resize the
image WITHOUT constraint proportions to 720 x 576. You can now check the
image with "Image -> Pixel Aspect Ratio" command.