UPDATE December 13, 2020: Currently there are no magical way to get images uploaded. The only way to make something appear as "new" is to change the 'Posted' date of some old images as explained on this old post of mine.

Hover the mouse cursor on right side next to 'Posted on...' or 'Photo taken on...' date. Click on appearing '(change)' link. Then change the date as you wish on the opening dialog box.

For example you may wish to change the date 'Posted on...' to make some older upload appear on your contacts' 'News'again. That trick is perfectly alright to do, to for example make some useful blog article or image appear again on 'News' after doing some updates on it. Moreover, this trick is useful if you first upload image as private, and then later on wish to turn it public and appear on 'News'.

To change date when 'Photo taken on...' is useful if for example you have forgot to correct your camera's calendar, or you have scanned some old printed or film media images of your's.

Update by Bernhard (Bergfex):

Please note that re-posting has no influence on the placement in the gallery.

This is because the gallery does not use a date stamp, but the sequential number of the upload, which is entered during the upload and can never be manipulated.

For this reason, old images will never appear in the gallery if you move them to the front of your photo stream by changing the 'date posted'.

Update concerning the group contributions

Moreover, there is no need to worry about the order of group contributions, if and when the 'Posted' date becomes changed. All the contributions and their order within the groups remains as the same.