Dont worry, you can recover deleted pictures after accidentally format the system, here's a good format recovery guide that helped me last time
[URL=http://www.asoftech.com/articles/recover-deleted-files-after-format.html|how to recover deleted files after reformat]
You will need to connect the Nintendo DSi to computer, and then download this [http://www.asoftech.com/apr/|photo recovery software].
hope this helps.