iMovie won't let me drag and drop images from My Movie

There always are changes as software evolves. I have never made a time lapse movie with the older versions. So I don’t know whether any of the changes are causing the issues that you are seeing.

Time lapse video can involve thousands and potentially more photos. That can overly stress the software and might cause some lack of smoothness. This also can happen if you have insufficient applicaton space for iMovie to function efficiently. A slower CPU might also cause it. Possibly also the issue may be confined to just this one project, and may not occur if you place your media in a newly created project. Thus it takes some trouble shooting and experimentation to find a solution, if one exists.

— Rich

You might try the simple procedure of deleting preferences, as that sometimes cures misbehaviors in iMovie. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

You might also try putting your photos into a newly created project and see if the time lapse works better there.

-- Rich

Is it possible that your images are in the heif format, that was not supported until High Sierra?

-- Rich

Great! Glad you found the answer.

— Rich

You are welcome. If you have time, post back with your findings.

-- Rich