How recover photos after format system memory DSI?

Formatting won't make the photos on memory card or other disk erased permanently. Those formatted files are still stored on the card just invisible or inaccessible. Only the FAT or NFTS table erased the information about where those data contained in photos were saved but the real data are still intact there in sectors of the card disk. As long as those data are not overwritten by new data, it's highly possible to recover them.

In your case as with recovering photos from your DSI memory, it is advisable to try some photo recovery software. Take the free trial version first.

Attention: before the formatted photos are recovered, do not save more data to the memory in case the formatted data are overwritten.