To elaborate why you get that "Clipboard empty" message.

CC3+ doesn't embed images in the drawing, but link to external files. This is why you can't simply paste from the clipboard, since the clipboard isn't a file CC3+ can refer to.

The reason it is reported as empty however, is that the Windows clipboard is kind of smart. It can keep the same data in multiple formats (for example, if you copy some text from a web page, the clipboard will store both a plain text version of the text, as well as a formatted version). Whenever you paste, the clipboard will pick the version that is suitable for the program you try to paste into. However, the only thing CC3+ can accept are either plain text, or CC3+ entities, and after copying an image from somewhere else, the clipboard won't contain any of those. Therefore it is empty as far as CC3+ sees, because it doesn't have anything to offer that program.

You can insert any image in jpg or png format into CC3+ using Draw -> Insert file. Note that this inserts a reference to the file on disk, so if you move/rename/delete the file it will stop showing up properly in CC3+.

Make sure to put it on a dedicated sheet, and then you can move that sheet up/down in the sheet order when tracing if needed, or just hide the sheet.

Embedding should work fine as long as you give it a nice clean Twitter link to the tweet:

I would probably do the coastline differently. That blue color doesn't look right when it ends up on the top of the cliffs like that. Maybe a different color, or maybe simply drop it completely. If you keep the "cliff mountains" on the same sheet as the landmass, you could use a glow effect for the coastline that would then trace the actual outside of the combined shape. Another option is to draw the coastline manually as a separate entity, and either skip it at the cliffs, or draw it at their base.

The reason you can't simply change the fill is because they are symbols. Changing the properties of a symbol won't change the entities it is made up from. (May feel weird for a simple symbol like this that is just a single entity, but symbols are often made up of many different entities with different purposes, so just changing them all doesn't make sense). Symbols needs to be exploded before you can change the properties.