Did you try clicking on the add picture icon? It looks like a mountain range with the sun above it near the bottom row of editing tools.
The other way that should work to add images is to just do a copy/paste, so you go to image you want on a website and copy it. Next, do a paste on KidzTalk.
We currently don't support embedding any videos, but you can always add links to anything. Just use the link tool (looks like a chain link in tools) or copy/paste it.