As you haven't identified your device, I can only go through the procedure as it applies to my HTC devices.
Longpress any image in gallery, an action page opens, the last action box is 'details', tap the details box, the details page is presented with a text box at the top. Tap in the text box and name/rename the image.
Hope this helps, but if not then please resubmit your question with more info eg. device make/model, for more assistance.
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