Very new here not just in the forum but also to the software,
I viewed just about all tutorials. 
I inserted clips made a couple overlay transitions by just overlapping the clips.
Then added some audio and tried a playback.
Up came INVALID not sure why or what is invalid?
Maybe someone can help.
Lets see if I can drag the capture in here.
Thanks
Hi there
Did you manage to sort your problem?
I have the same problem. I started to make a very basic slideshow with pictures and an audio file background, and on opening the project file to continue working on it tonight, the audio is playing but the pictures all says INVALID. I have not moved or deleted them.
Can anyone help please? Im running Shotcut 17.11.07
Thank you
Ronnie
Hi !
It’s because your picture is in ".png"
Use only “.jpg” 
Audrey
Shorcut supports .png files. I recently used them in a slideshow.
-=Ken=-