Many thanks Herve, Not too sure if I am doing this correctly, maybe you can advise please. I resize the image to 600pix max and then add the borders afterwards which then makes the image 638ish pixels. Which would be the best option?
This should do the trick:
1. work at your image at maximum size and save it regulary
2. when you are happy, decide of the size of the border (e.g. 10 pixels)
3. resize your image to 580 pixels and make the last adjustements (sharpen ?)
4. add your border, signature, ... you should reach exactly 600 pixels
5. save and upload
Hope it helps,