Is it safe to delete thumbnail images? Will doing so delete main images? Can't find this topic anywhere. Thank you.

No. When Photos imports a photo into the library it creates a thumbnail/preview version to display in albums and other windows of the library. Deleting the thumbnail/preview also deletes the full sized original file. There's no way to separate them when deleting. Just consider them one and the same.

Where did you drag that file from? Can you take a screenshot of one on the Desktop and include it in your next post?

I'm guessing that you're opening the image file with the Preview app. That's why is says Preview at the top of the window. What you're calling a thumbnail is actually an image file with a thumbnail image as its icon.

So when I click on the desktop thumbnails it only says preview

I'm not sure what that means. When you open a JPEG file by clicking it, you will see a preview, but that does not mean tat the image file is a preview. Does the filename has "preview" in its name? And other files have "thumb" in its filename? Then you may be seeing fragments of an iPhoto or Photos library that you dragged out of a library.

Are your Photos library and iPhoto library still in working correctly? Do you get any warnings about missing photos when you are working with iPhoto or Photos? If not, I would copy the thumbs and previews you are seeing in on your Desktop to an external drive to keep them safe, until you know for sure, that you are not missing any items from your photo libraries.

You have been moving files from a search for all photos on your Mac.

You screenshot is showing a search for each and every item on your Mac with the word Adobe in the name or the path to the item.

This may have removed photos needed by Adobe software from the application support of the particular program. For the particular thumbnail you selected in the screenshot you can see the original place in the path bar at the bottom. It is a file from ~/Library/Containers. this is the sandbox, where running applications exchange data and the files there can be recreated. and is safe to delete.

But the search could easily have shown you files and folders in other caches or inside applications, that are needed to run the applications. You must not delete the items, before you now if they are needed. as I said before, move them to some folder "dragged items", to keep them safe. Preferably use a folder on your system drive in your home folder, and drag the items there, so you do not copy them. The problem is, that is impossible to tell, where you removed them from, because you started with an "All Photos" search. And the saved search seems have to lost the focus - as far as I can tell, this folder in question does not have Adobe in its name.

Never delete or move photos from a Finder search window, at least, without looking at the path bar at the bottom, so you know exactly, what you are deleting.

As long as you just drag files between folders on the same disk, the system may find them again, if they are missing from some folder. So moving them to some folder on your system drive may take care of missing items in system folders.

If you still encounter problems with missing items, when you run some application or open a document, it will be safer to restore your Mac from the last backup you made before you dragged thumbnail images around. The thumbnails now can be missing from each and every application support folder on your Mac, from Pages or Powerpoint documents, from mail attachments etc.

Are you seeing the thumbnails in the Finder? Or are you seeing in Photos in the Moments large and small photos side by side because you imported accidentally thumbnails as photos?

Good luck, Martha Anne. Feel free to post back, if you notice problems because of missing photos in some application.