How to delete pictures permanently after factory resetting done on Android phone

You may use the following apps for the purpose of permanently deleting your files from your Android smartphone -

1. Secure Delete

This is a small app available on Play Store. It let's you select the type of files you want to delete. You can selectively delete files based on what kind of files you want to delete. If you are removing photos, be sure to delete the thumbnails as well (in fact, the app prompts you to do that).
Deletion takes a lot of time, so be patient.

2. Secure wipe

This app cleans up the free space. Once you have hard reset your device, install Secure Wipe . Select the options for sanitising system space, SD card and the optional SD card and tap on Start Wiping. Be sure to charge your phone fully or keep it on charging because this deletion takes a lot of time.

Also make sure, you have deleted all your accounts on the phone. Sync with Google+ or such other services may restore your photos back.

Find file shredding app and use it to delete the previous data. The best thing can be done in such cases is also remove the SD card.

If the SD is separated from the phone, you can get the idea of the internal storage images and external one. You can then decide which data to erase more. And you can also shred the files from the external SD card using PC software or app. This can be much easier.

Avoid using the app, that does overwriting of the data for deletion, because that will take entire space of your SD card. And this method is not good at all. If you have to overwrite the data to wipe the SD card then better add other non important files yourself.