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Go to your front page (/yourusername), click "See all my posts" below your newest pictures. Drag and drop to the top the ones you wish to feature to the amount you set up in your settings.
Make sure that the sum of all the files are you are uploading at once does not exceed 30MB (3MB for non-subscribers). If you are having troubles, try uploading your images one by one. If the problem still persists, try our recommended browser: Chrome.