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can I do it in paint?
Yes, but there are better program to do it in, like
Gimp (Free) or Photoshop.
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and the picture of footer and header needs to be as width as the box
Normally, you have the width of the content, banner (header/ top) and footer (bottom) all the same, it just wroks better that way.
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where can I see the px of a picture at designtime?
In paint, I don't believe you can, in Gimp it is at the bottom left of the canvas screen.
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Inside the corners I need to fill the picture with the boxcolor en outside the corners, need to fill the corners with the bgcolor af the mainpage? Correct?
Inside of the corners needs to be the color you want the rounded rectangle to be, however if you use Gimp or Photoshop you can forget about the outside of the corners as you can set it to transparent, save as .png and have the transparency kept.