I mean, I've been here and I've doing stuff here for almost two years, but I have never known this one. And I don't know if this is already asked, but how do I change the size of an image? Is there something I should insert the code or something I should change?

You can use "Paint".Insert your image there and press resize or something that have "size"words.then Un-checklist a option that have a "Maintenance"words.then you can choose between Pixel or other.and type the dimension you want.

If you talking about non-flash stuff
I used this program to resize any image files, it's simple to use
https://imageresizer.codeplex.com/

you should use this code, i don't know if it works [IMG WIDTH="value"] and end it with [IMG]

You can use Imgur to resize images when you upload them, you also have cropping and rotating tools and the like.