I have just copied a picture from the internet, and I have successfully pasted it on an email message and sent it. This is what I did:
1. open email 'compose' page, so that I didn't have to faff about when I had the picture ready and waiting.
2. right-click on picture.
3. on drop-down menu, left-click on 'copy'.
4. call up email 'compose' page.
5. press and hold Ctrl key, then press V.
6. picture appears on email 'compose' page.
7. click 'Send' and picture is sent.
Have to say I'm not coming up with any obvious reasons, but reiterate you should try closing stuff down to see if it frees up resources and starts to work, or the old favourite, a reboot.
The clipboard = used for copy and paste - can fail if, as Old_Geezer say, system resources need freeing up (these things sometimes just happen after using a computer for a while); the simplest way of getting everything back to normal is a reboot.