Here you go ^~^

First question: "how to make a link change to a picture"

It's actually quite easy, once you know it :D

So first you add your picture just like this (I guess you know how to, but just to be sure it will work):

press on the "image" button:

This is what must appear right now:

Past the URL (I always use imgur to upload pictures on, because imgur gives you an URL):

Press on the "ok" button:

So far we have our picture:

Okay! Here we are! Now let's add a link to it ~ I'm just going to choose a random link, so don't mind that :P

So now you select your picture:

Press on the "link" button:

Now this must appear:

Just copy the link you would like the picture to make a link to and past it under "URL":

Now just press on "ok"

Normally, when you go back to your story (not when your editing), a tab with your linked page will appear when you press on the picture~

Like this (press on picture):

Told you it was easy :3 The next question is a bit more difficult

Question two: "how to make a small video from youtube in a blog or stories"

So this is what you do:

First thing is go to youtube (I'm going to make a small video from f(x)'s song 'Rum Pum Pum Pum' as example) When you scroll a bit down you'll see this:

Ok, now press on "Share":

Now this must appear:

Now press on "Embed":

Now you see this sort of link:

Don't copy the whole link, only the //www.youtube.com/embed/xnku4o3tRB4 for this video. This goes for every youtube video, only this part is or every video different. Only copy this:

Now copy this part and go back to your aff story tab and press on the "Iframe" button:

Now this must appear:

Now past the part you copied:

You can choose the width and height in which the video will be shown. I prefere to take width: 350 and height: 200, but it's up to you :D :

Now just press on "ok" and here you go:

I hoped it helped you a bit, and if there's something you don't understand feel free to ask~ xoxo