Your device should be having an inbuilt file manager, if it doesn't you can download from Play Store. Many are available. Solid Explorer is a good one. Photos are stored in /sd/DCIM or if you have downloaded videos/ photos, they would be in /sd/downloads. You will need to search for the location and delete them, if stored elsewhere
If you don't wish to identify the folders, use Solid Explorer, on launching the application , tap the menu button on left and scroll down where you will find photos and videos folder. Browse through them, select and delete
Alternatively, DiskUsage gives a good visual feedback of your storage and let's you select the file you wish to and delete
