For the gallery, it's not possible to have a signature without adding it in each time.
If you are referring people to your blog for more details on your upload, SCS does request that you link to the direct post rather than a generic link
. It makes it so much easier for people clicking on the link
To edit your recent upload ~
1. Go to the gallery page for your upload. Click on the words "edit photo" in small writing toward the right, between the photo and the comment input box.
2. That will take you to a screen similar to the upload screen.
You can make any edits you need to here, including changing the photo.
3. Click on "submit changes" at the bottom of the screen.
It may take a little while for the gallery to process the changes, so don't expect to see them instantly. It can take twenty minutes or so, and if you're not a fan-club member it could be longer because cards go into the moderation queue for approval.
Hope that helps. If you need more help, just ask!