This is the the thread where the own affected user explains the Photoshop issue and how it can be solved: High sierra 13.13.3 OS. Problem with thumbnails not showing images, only generic jpg. But it's only useful if new files are created my problem is that I already have lots of files that I need to preview. Thanks in advance.
How to Change Where Screenshots are Saved on a Mac. There are two ways to change the default screenshot save directory on a Mac, depending on your operating system. The easier way is through the Screenshot app in macOS Mojave. For mac OS High Sierra or earlier, you have to use Terminal, an app for entering command prompts to control your Mac. I had this problem with High Sierra, Preview, and a DisplayCal profile. The latest version of DisplayCal, 3.5, has in its release notes. Mac OS X: Set default profile type to single curve + matrix with black point compensation due to long-standing Mac OS X bugs with any other profile type.
Preview User Guide
You can use the editing tools in the Markup toolbar to mark up an image file, providing feedback or pointing out something you want to remember.
If the image is on a page in a PDF file and you want to comment on only the image, you can extract the image as a separate image file. See Extract a PDF page as an image for instructions.
If you only want to comment on part of an image that you’ve opened in Preview, you can select part of it, choose Edit > Copy to copy just that part, then choose File > New from Clipboard to create a new file. You can also crop the image to remove areas that you don’t want.
Note: If you annotate an image file, you can’t edit, move, or delete the annotations after you save the image. If you think you’ll need to change annotations later, you can convert the image file to a PDF file, then add annotations to the PDF.
Mac Preview App For Windows
In the Preview app on your Mac, click the Show Markup Toolbar button (if the Markup toolbar isn’t showing).
Use the tools in the toolbar to mark up the image (or use the Touch Bar).
Tool
Description
Rectangular Selection
Use any of the following shape tools to select an area in an image and then crop, copy, or delete:
Rectangular Selection
Elliptical Selection
Lasso Selection
Smart Lasso
See Extract an image or remove a background.
Instant Alpha
Remove the background from an image.
Sketch
Sketch a shape using a single stroke.
If your drawing is recognized as a standard shape, it’s replaced by that shape; to use your drawing instead, choose it from the palette that’s shown.
Draw
Draw a shape using a single stroke. Press your finger more firmly on the trackpad to draw with a heavier, darker line.
Note: Unlike the Sketch tool, the Draw tool doesn’t recognize standard shapes.
This tool appears only on computers with a Force Touch trackpad.
Shapes
Click a shape, then drag it where you want. To resize the shape, use the blue handles. If it has green handles, use them to alter the shape.
You can zoom and highlight shapes using these tools:
Loupe : Drag the loupe to the area you want to magnify. To increase or decrease the magnification, drag the green handle; drag the blue handle to change the loupe size.
Highlight : Drag the highlight where you want. To resize it, use the blue handles.
See Rotate and modify shapes added to a PDF for more options.
Text
Type your text, then drag the text box where you want.
Sign
If signatures are listed, click one, then drag it where you want. To resize it, use the blue handles.
To create a new signature:
Using your trackpad: Click Trackpad, click the text as prompted, sign your name on the trackpad using your finger, click any key when you’re finished, then click Done. If you don’t like the results, click Clear, then try again.
If your trackpad supports it, press your finger more firmly on the trackpad to sign with a heavier, darker line.
Using your computer’s built-in camera: Click Camera. Hold your signature (on white paper) facing the camera so that your signature is level with the blue line in the window. When your signature appears in the window, click Done. If you don’t like the results, click Clear, then try again.
Using your iOS or iPadOS device: Click Select Device to choose a device (if more than one is available). On your device, use your finger or Apple Pencil (on iPad) to sign your name, then click Done. If you don’t like the results, click Clear, then try again.
Your devices must meet Continuity system requirements. See the Apple Support article System requirements for Continuity on Mac, iPhone, iPad, iPod touch, and Apple Watch.
See Fill out a PDF form.
Adjust color
Change values for an image’s exposure, contrast, saturation, tint, and more.
To have the values automatically adjusted, click Auto Levels. To return to the original values, click Reset All.
Adjust size
Change an image’s size and resolution.
Shape Style
Change the thickness and type of lines used in a shape, and add a shadow.
Border Color
Change the color of the lines used in a shape.
Fill Color
Change the color that’s used inside a shape.
Text Style
Change the font or the font style and color.
Annotate , ,
Annotate the item by sketching or drawing on it using your nearby iPhone or iPad . If both devices are nearby, click , then choose a device. The tool may appear highlighted to show your device is connected. To disconnect your device without using it, click the tool again.
Your devices must meet Continuity system requirements. See the Apple Support article System requirements for Continuity on Mac, iPhone, iPad, iPod touch, and Apple Watch.
Mac App For Bigger Image Previews In Sierra Vista