When you import an image or anything else, the browser has a category (second one from the top on the left) that is your computer user name. (i.e. "Andrès", "Home", "Computer" etc.) When you click it, it actually brings you to the (user)/Documents/ folder. I believe it should be appropriately labelled "Documents" with an appropriate icon as well. I believe it was named Documents in older versions of Scratch. (or just Scratch for Windows)
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It's also on Windows.
I support this, but they might not use a special file dialogue in 2.0, as all browsers come with their own.
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Thanks for the support. For the online version of Scratch 2 it seems that it uses the system-specific browser pop-up, but for the downloadable client it might still use the classic style. Personally, I'd rather have the system-specific one as it is much easier to navigate. (I also believe that if you drag in a file from an area like the desktop to the system-specific browser on OS X + Windows, it will navigate to that file in the browser without digging through directories.)
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