I'm using Android 4.1.2 (I tagged with 4.1 jelly bean because there is not a 4.1.2 tag, my phone is Samsung galaxy S3 mini. If I open my gallery, in wich all pictures on the phone should be present, only pictures on picasa and pictures taken by the camera do show in the gallery. I have a lot of pictures in my phone, for example pictures on the downloads folder. They don't show up.
I tried with the answers of other questions and it was in vain.
I restarted the phone many times. I re-scanned the device with this app: https://play.google.com/store/apps/details?id=com.promethyl.force.media.scanner&hl=en
I went into settings, manage applications, all apps, I stoped the media storage app and the gllery app, also deleted all data (and thus the cache, in fact after deleting the data I tried to delete the cache but the phone didn't let me, I realized that it was because when deleting all the data, It also deleted the cache)
Also music on the phone do not display in the music application. I also deleted music app data, but still it's not there.
EDIT 07/09/2015: The solution that worked for me was to delete .nomedia "hidden" files, in particular a .nomedia file in the MAIN folder preventing the media scanner to scan anything in the main folder of the SDcard. After deleting it, all pictures everywhere seem to be scanned and they appear in the library.