track and field 100m style scrolling background

what would be the best way to achieve this?
i cant quite figure what the maximum length of the background image can be?
at the minute im going for a tiled approach but im having trouble figuring out how to get the next tile to appear at the end of the one before it
i cant quite figure what the maximum length of the background image can be?
at the minute im going for a tiled approach but im having trouble figuring out how to get the next tile to appear at the end of the one before it

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not being able to work out how to just get the tiles to scroll next to each other is killing me lol