Screenshot > Message App doesn't connect on iMac

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The following steps provide guidance on how to capture a screenshot, and the steps you can take to share it once captured:

Take pictures or screen recordings using Screenshot

1: On your Mac, press Shift-Command-5 (or use Launchpad) to open Screenshot and display the tools.

2: Click a tool to use to select what you want to capture or record (or use the Touch Bar).

For a portion of the screen, drag the frame to reposition it or drag its edges to adjust the size of the area you want to capture or record. 

3: Click Options if you want.

The available options vary based on whether you’re taking a screenshot or a screen recording. For example, you can choose to set a timed delay or show the mouse pointer or clicks, and specify where to save the file.

The Show Floating Thumbnail option helps you work more easily with a completed shot or recording—it floats in the bottom-right corner of the screen for a few seconds so you have time to drag it into a document, mark it up, or share it before it’s saved to the location you specified.

Start the screenshot or screen recording:

• For the entire screen or a portion of it: Click Capture.

• For a window: Move the pointer to the window, then click the window.

• For recordings: Click Record. To stop recording, click the Stop Recording button in the menu bar.

When the Show Floating Thumbnail option is set, you can do any of the following while the thumbnail is briefly displayed in the bottom-right corner of the screen:

• Swipe right to immediately save the file and make it disappear.

• Drag the thumbnail into a document, an email, a note, or a Finder window.

• Click the thumbnail to open a window where you can mark up the screenshot or trim the recording, or share it.

Depending on where you chose to save the screenshot or recording, an app may open.

You can find additional information and other methods on the following page: Take screenshots or screen recordings on Mac

We hope this helps!

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I had never tried it before, but I can confirm that choosing Messages as the destination is NOT working for me, either. Sorry, no solution to do it directly, but it is easy enough to send it to Preview, or to turn on "Show floating thumbnail" and share from there. In fact, you can share from there not just to Messages but to specific people or groups in Messages. One extra step at the start but one less in the end evens it out, I'd say.

Re check what's chosen as the Destination for Screen Shots

refer to image below and make sure Messages has indeed been chosen

Ouch - you are Right 😔