Choose the size of the picture .... I really don't know !

Hi everybody
I still have got a problem- How can I choose in the url the size of the image I want to print in the browser
I have defined different possible size of one image in my album edition.
One is 150X150 px
another one is 280X280
another one is 500X500
What do I've to write in the url to choose the size 280X280 for ex
www.mywebsite.com/gallery/v/album/280X280
or something like that ?
Thanks for your help

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Sapristi