If the pics were on the SD card, get a card reader, put the card into it, plug it into your computer, download
PhotoRec - CGSecurity, burn it to a CD, boot your computer with it and, if there are any blocks of data on the card that look like pictures, PhotoRec will recover them. (It doesn't care about operating systems, directories, etc., just data.)
If the pictures were on the phone's internal storage you have two problems.
1) You can't recover deleted files unless the phone is rooted.
2) You have to install a recovery app on the phone.
The problem with both of them is the same - you'll be writing to internal storage, so you may overwrite the pictures you're trying to recover. (Disk Digger is free for pictures.)
Remember an old computer addage - any file you don't have backed up to at least 2 independent devices is a file you don't need. (A cloud storage account counts as a device these days, so backing up to Dropbox and Google is backing up to 2 devices. The odds of your phone, Google, Google's off-site backup, Dropbox and Dropbox's offsite backup all going bad at the same time are ... well, the end of the world could cause that, but not much else. So even if you drive over your phone with a tank and Google crashes, you can still get the files from Dropbox - and back them up to 2 other devices.)