First thing to do is stop using the device! When things are deleted they're not actually removed. Instead the operating system just marks the File Allocation Table to show the part of the disk/memory card the file occupies isn't used. The data - including pics - is still there. Until it gets overwritten, that is.
Using another machine, Google 'Photorec'. There are a number of links to the software. Download & install it. If you can, remove the SD card from the phone and mount it in a PC. Run Photorec against that SD card and it will find all files that are recoverable, even if the FAT shows the file doesn't exist anymore. Photorec looks for the raw data of the file, not the pointer in the FAT.
If you can't remove the memory card then you need to find some way of accessing the data partition as 'root', otherwise the operating system hides stuff from you. In Android you can enable root access when you enable developer access. You will have to google your phone to find out how. Once you have root access, connect it to a PC, mount it as an external drive and sic Photorec on to it.