Some apps and frontends don't have that field yet, but the main web interface should have it when editing posts: https://lemmy.ca/
I'm not sure what caused it to not show up in the first place, but that's how I fix thumbnail issues in my posts
Lemmy also lets you manage your uploads now if you wanted to upload it again / replace it. Go to your profile and there should be a tab for it. You should also be able to access it here:
https://lemmy.ca/u/mr_stevenson_ii?page=1&sort=New&view=Uploads