For pictures, you need to get the URL and then stuff and stuff, bleehhhh hard to explain xD
For videos, you can either just leave the link in your signature, or click that picture of a piece of film next to the + sign on the top and insert the URL to the video. That will show the whole video.
And for spoilers, click the + sign. A drop down menu will appear. Click "Spoiler" (which has a flag next to it) then insert the name of your spoiler (optional. If you don't want to, you don't have to) then click continue. After that, just insert whatever it is you want to put in the spoiler in between the "[SPOILE R]" and "[/SPOILE R]" thingys which are now in your signature.
And don,t forget to save everything ._.
Edit: Damnit, it made it into a spoiler and I can't fix it. Ugghhhhh
Edit 2: wait nvm fixed it xD