I'm not usually able to help around here because of my limited knowledge, but I hope I can help here

If you have the amended picture on your computer ready to upload, then make sure it has the same filename as the existing photo. Then all I usually do is go into the ftp and into the appropriate folder, and put the new picture into the folder. It will ask if you want to overwrite the existing picture so click yes, and it will overwrite the picture but should keep all the comments and the same 'date added' as the original picture.
Alternatively, in your control panel, go to edit images and find the picture you want to edit. Replace it with your new picture but don't alter the date on the edit page. It should keep the same comments and hits as before.
Hope I helped