Computer Help and Support

Using your favorite file browser app:
Navigate to the SD card, create a folder for your pictures. (Same name as the internal storage helps.)
Copy, not move, the image files from the folder on the internal storage to the folder on the SD card.
Verify that the copy was successful.
Restart the phone to flush the file buffers and kickstart the media indexing service.
Again verify that the copy to the SD card was successful.
Delete the old files from the internal storage.

Your camera will probably store is images and videos in the "DCIM" folder tree.
Social media apps like to use subfolders in the "Pictures" folder.
Some apps like to use the "Download" folder.

While you are at it, there is also the "Music" folder and the "Documents" folder.

Some apps that use lots of data space (camera, maps, GPS, games) sometimes let you store the data on the SD card while leaving the app on the internal storage. Check the settings in the individual apps.

Also keep in mind that having a large number of files in a single folder is not a good idea (slows down the performance).