Simply you should use some editor to turn these to required orientation. The good practice is do that when you optimise your images for web. You can use any editor that is included on your OS to do this job or use any editing software or online services.
Thanks a lot for your help !
Found the same article and it is indeed the best solution, i’m editing them in PS and saving them as PNG. Seems to be a good solution since it solves the problem.