Hi, Sylvia,
Go to your Galleries tab and click any gallery to open it.
Once inside the gallery, click the 'Edit This Gallery' button at the top of the gallery.
From there, just click on the 'Position' button in the row across from any image, then click the little down arrow and choose the new position for your image.
Hope this helps! :-)
EDIT -- Or, based on Jeffrey's response, I completely misread your question. Good times! ;-)