When developing media for a speech, the speaker's responsibility is always to not plagiarize! The speaker must know that just because you change a photo, video or audio file a little bit here and there, that that does not mean the work now belongs to you. The owner of the original material still must get credit for their work, as they were the one who originally created it and made it possible for you to even have something to edit. In the past, many singers have gotten sued for having songs that sound similar to other artists songs. Just because the singer has taken the music and made the song their own does not mean that they do not need permissions from whoever originally made the song. The point is, unless you made it completely from scratch, it does not really belong to you even if you have altered it heavily. Citing your work and citing your sources is really important in order to avoid academic dishonesty or even legal troubles for once you're out in the real world.