Covering edited video and audio with stills

I'm trying to do something that, in my mind, should be simple, but it's proving difficult.

I have edited the audio of my project. This consists of some video (which appears in the Video 1/Audio 1 timeline) and lots of music, which I've put alternately in the Audio 1 timeline and the Music timeline, so that I can do mixes and fades between the music. This has worked nicely.

Now I just want to cover the whole thing with lots of still images. But when I try to put a still image in, for example, Video 2 timeline, this starts affecting the music clips in the music timeline, creating white gaps inside music clips. This, of course, messes up the whole audio edit I've already done.

I just want to slap lots of still images on top of the edit without effecting anything I've already done.

Anyone know what I have to do, please?