It is sad that this functionality is currently not available. You have to click the 'Photos' icon in the RTE and upload the image/sceenshot into the post, or drag and drop the image file onto 'Choose File' as an attachment.
You CAN do this! It's not that obvious how to do it, but once you learn, it will save you a ton of time.
(1) Take your screenshot, and make sure it's on your clipboard
(2) In your post on your Lithium community, bring up the photo-uploader
(3) In the photo-uploader modal window, click on the right side (the white bar), then click into the grey editor area. Then Cmd-V to paste
That's it! Your screenshot will paste in as an image, and you can add it to your post! Sometimes it takes me a couple rounds of clicking around the photo-upload modal to get it to work, but it does.
I took a tiny video to try and explain what I mean by clicking on the right side and then the editor area (uploading as a gif since I apparently can't upload videos here; hope it works!):