When image thumbnails stop showing on a Mac and how to get them back

A folder of photos showing plain grey squares instead of the pictures themselves, or a file that opens to a blank white pane the moment you press the space bar for Quick Look, is one of the more common small annoyances on a Mac. Nothing is actually broken in the way that word usually implies. A thumbnail is a preview macOS builds and stores separately from the file itself, and previews are the kind of thing that goes missing without the file underneath ever being touched.

There are four usual causes, and they are worth checking in this order: cheapest and most common first, so you are not resetting a system cache when the real answer was a single checkbox.

Is icon preview switched off for this folder?

Finder's Icon view has its own setting for whether it draws a small preview of each file or just its generic file-type icon, and that setting can be turned off per folder. If one folder shows thumbnails and another does not, this is almost always why.

1. Open the folder. Click into the folder where thumbnails are missing, and make sure it is set to Icon view.
2. Open View Options. Choose View, then Show View Options in the menu bar, or press Command-J.
3. Tick Show Icon Preview. If the box is unticked, the folder is showing generic file icons on purpose. Tick it and Finder redraws the thumbnails immediately.
4. Click Use as Defaults, if you want it everywhere. This applies the same setting to any folder that has not been customised on its own, rather than just the one you have open.

Apple's own guide to changing how folders are displayed in the Finder covers the full View Options panel, including this checkbox and how the Use as Defaults button works. Very small icon sizes can also make a preview harder to make out even when it is switched on, so it is worth dragging the icon size slider up a notch before deciding the setting has not worked.

Reset the Quick Look thumbnail cache

Every thumbnail and Quick Look preview on a Mac is generated and stored by a background process called quicklookd. Most of the time it is invisible. Occasionally, usually after a macOS update, a crash, or the Mac simply running for weeks without a restart, its cache gets out of step with the files on disk, and Finder keeps showing a blank icon for a file that is perfectly fine. Terminal has a built-in command for exactly this.

1. Open Terminal. It is in Applications, then Utilities.
2. Run the reset command. Paste qlmanage -r cache and press Return.
3. Give it a moment, then browse back to the folder. Nothing appears to happen in Terminal itself; the command clears the stored cache and quietly rebuilds it as Finder asks for each thumbnail again.

This command only clears a cache. It does not touch, move or delete a single one of your files, and there is nothing here to undo. The trade-off is speed rather than risk: clearing the cache means every thumbnail has to be regenerated from scratch, so a folder with a few thousand images can feel slower to redraw for a minute or two the first time you open it again. That is expected, and it settles down once the cache has rebuilt.

Has the file actually finished downloading?

A thumbnail can only be generated from a file that is actually present on disk, in full. If the image lives in iCloud Drive, Dropbox, Google Drive or a network share, and only a placeholder has been downloaded so far, Finder has nothing to build a preview from and shows a blank icon or a small cloud symbol instead.

The most common version of this on a Mac is iCloud Drive's Optimise Mac Storage setting, which deliberately keeps older or less-used files stored only in iCloud and swaps in a lightweight placeholder locally to save disk space. Apple's own page on storing files in iCloud Drive on Mac explains how it works. Click the file to trigger a download, or turn Optimise Mac Storage off under Apple ID, then iCloud, in System Settings if you would rather keep everything stored locally all the time. The same idea applies to any other cloud-synced folder set to save space rather than keep a full local copy, and to a slow network drive that simply has not finished transferring the file yet.

Is the file something macOS genuinely cannot read?

Once the first three checks are done, what is left is a smaller and more honest category: files macOS truly cannot generate a preview for. An image format Quick Look has no built-in support for, a RAW file from a camera released after your current macOS version shipped, or a file that was interrupted mid-transfer and is quietly corrupted, all show the same blank result as everything above, with none of the same fix.

The quickest way to tell the two apart is to open the file directly in Preview rather than relying on the thumbnail. If Preview opens it normally, the image itself is fine and the problem really was the cache or the view setting. If Preview also fails, or shows an error, the file itself is the issue, and no cache reset will change that.

Thumbnails are a cache, not a promise

It is worth being honest about what a thumbnail actually is. It is a preview the operating system builds on the fly and stores separately from your files, precisely so it can be thrown away and rebuilt whenever it needs to be. That is a sensible design for a general-purpose file browser, but it also means the preview can go missing for reasons that have nothing to do with the file itself, which is most of what this article has been about.

A library that keeps its own preview of everything you have saved, rather than asking the system to regenerate one on demand, simply does not run into this particular problem. Muse stores a preview alongside every item you add to it, so a resized reference or an old screenshot still shows as a picture rather than a grey square, independent of whatever Finder's cache is doing that day. It will not fix a genuinely corrupted file, and it is not a reason on its own to reorganise a Mac. It is simply one less place a preview can quietly disappear from.