How to paste an image instead of a file on a Mac
You copy a photo from Finder, switch to a document or a message, and paste. Instead of the picture appearing in the text, you get a file attachment, an icon, or a preview thumbnail that is clearly the file rather than the image inside it. The picture and the file that contains it are two different things to a Mac, and the paste took the wrong one. Getting the image itself is easy once you know where the pixels actually live on the clipboard.
Why pasting a file gives you a file
When you copy an image in Finder, you are copying the file: a reference to a document on disk that happens to hold a picture. The clipboard records where that file is and its name, but it does not open the file and pull the picture out. So a destination that accepts files, like an email or a chat upload box, takes the file and attaches it. A destination that wanted actual image data has nothing to draw, so it falls back to showing the name or a small preview.
To paste the picture inline, you need the image data on the clipboard, not the file reference. That means copying from something that has already opened the image, rather than from Finder.
Copy the image, not the file
The most reliable route is Preview. Double-click the image so it opens in Preview, then choose Edit and Copy, or press Command-C. Now the clipboard holds the picture as image data. Switch to your document or message and paste, and the image appears in place, because you handed the destination pixels instead of a file on disk.
The same idea works anywhere the image is already open and on screen. If you can see the picture rendered in an app, copying from inside that app usually copies the image data. If you are copying from Finder, where the file is only listed and not opened, you tend to get the file instead.
Other ways to get image data on the clipboard
There are a few shortcuts worth knowing.
Take a screenshot straight to the clipboard. Press Command-Control-Shift-4 and drag a box around the picture. Instead of saving a file to the desktop, macOS puts the captured image on the clipboard as image data. Paste it wherever you like and the picture drops in. This is handy when the original file format is awkward and you just want the visible image.
Copy an image from a web page. In a browser, right-click the picture and choose Copy Image, not Copy Image Address. Copy Image gives you the picture data, ready to paste inline. Copy Image Address gives you a link instead, which is a different thing again.
Use Quick Look as a viewer. Select the file in Finder and press Space to open Quick Look, and in many cases you can copy the rendered image from there rather than the underlying file.
Each of these has the same effect: the picture reaches the clipboard as image data, so the paste produces a picture rather than a file.
When you actually want the file instead
It is worth being clear about the opposite case, because sometimes the file is what you need. If you are sending the image to someone who has to save or edit it, keeping the original file matters. Image data pasted inline is often re-encoded and can lose the original resolution, colour profile, or format. An attachment keeps the file exactly as it was.
So the decision is the same one you make with any copy on a Mac. If the destination is going to display the picture in place, you want image data. If it is going to store, forward, or open the file later, you want the file. Copying from Finder leans toward the file. Copying from an app that has the image open leans toward the data.
Letting the destination decide
Choosing between the picture and the file every time is a small tax, and it is easy to get wrong because you pick before you know where the image is going. You copy the file, paste, see an attachment, undo, open Preview, copy again, and paste once more.
Ledge takes a different approach to the same problem. You drag the image up to the notch at the top of the screen and let it rest on a shelf without committing to a form. When you drag it off, the destination decides what it gets. Drop it into a chat that takes uploads and the file uploads. Drop it into a field that expects text, like a terminal, and it hands over the path. Drop it on Finder and it moves the file. You gather the image once and let each place pull the version it can use, rather than re-copying it in a different shape for every target.
That will not re-encode a picture into inline pixels for a document by itself, so for pasting a screenshot into a note the copy-from-Preview trick is still the direct move. Where it helps is when the same image needs to reach several different apps and you are tired of copying it a new way for each one.
The short version
Pasting from Finder gives you the file, because the clipboard holds a reference to the document, not the picture inside it. To paste the picture, put image data on the clipboard: copy from Preview, screenshot straight to the clipboard with Command-Control-Shift-4, or use Copy Image in a browser. Keep the file when someone needs to save or edit it, and use image data when the destination just needs to show the picture.