Currently it is not possible to copy and paste a picture from the clipboard into the chat when using the desktop app (tested this on linux and windows with version 1.2).
I would like to use this feature to send screenshots to my colleges.
Copy and pasting from the clipboard with either the GUI (e.g. right click into the message field in a delta chat 1:1 or group conversion should display the option paste) or with ctrl+v should place the picture into the message field so that you can add some text and then send it (basically just like when using drag and drop currently with a picture that resides on your filesystem, I find it strange that that feature is already present)
Using ctrl+v after ctrl+c does nothing, there is also no GUI option “paste” displayed when right clicking into the message field.
The signal desktop app for example does provide this feature for pictures (but not for other files). So maybe we can look at their implementation ?