You can't actually load a skin directly in Wurst. Wurst's skin stealer feature gives you the .png files of other players' skins, which are stored in the wurst/skins folder. If you want to change your skin, you should do it on the official Minecraft.net website.
If you are using a cracked account, you can refer to this older thread, which explains how to change the skin on a cracked alt: https://wurstforum.net/d/37-how-do-you-make-a-cracked-alts-skin-a-specific-skin
Keep in mind that not all features work with cracked accounts, and the best experience can be had with a genuine Minecraft account.
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