In Gallery you can long press the picture to select the file, then tap the three dots at the upper right, tap Details in the menu that is displayed, then scroll down to see the path of the file. If you place a file named .nomedia (contents irrelevant) in the same directory as the picture file, files in that directory should no longer be shown by Android media apps such as Gallery. In my case, this Pandora icon is bmwgroup/com.pandora.android/icon.png, and once I copied a .nomedia file into that directory (using My Files) the picture was no longer shown in Gallery.
Larry