The way i put pictures of pets is go to the archive
hold down on the pets image ( I'm on mobile so maybe try right clicking on the pets image ) select the option to open image on a new tab and then i go to the top of the image on the new tab and copy the link. From there i use [img]link[/img]
To get different font sizes and colours I'd suggest going to one of your threads like this one and looking at the options on the top bar when making a new 'post'. From there you can select things like font colour, size etc.
Your can learn some of the things off by heart like
bold [b]words[/b
center
[center]words[/center make sure to add the closing ']' i left it out so they would show
Don't actually post it on your thread just copy it and then paste in your trading rules :3
I hope that makes some sense :'0 I'm still pretty new to this to but i hope i could help in some way