如何在幻灯片中制作相同尺寸的所有图片？

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I'm trying to make a slideshow and I just copied and pasted this from w3schools.com. It worked, but the buttons disappeared and the pictures are all completely different sizes. Even if I do width:__ and stuff, it doesn't get fixed... Can somebody help me with this?I'm trying to make a slideshow and I jus