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You can edit this one until it is 7 days old actually. If you use the editor built into steemit, you can drag a photo and drop it into the editing field or you can hit "selecting them" at the end of the line of text about images in the bottom of the main editor field (between where you write the text and where the tags go). The thumbnail gets proportions that are 800x450. You can use a different size picture, but that is the "ideal" for the thumbnail.

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