Edit: I notice that it looks like you figured this out in one of your threads -- sorry for my late reply! >.<

Hey there @Invidii -- the way I personally do it is by going into my inventory, right-clicking an item and copying the image address. Though on some devices I don't have this option, so instead I'll open the image in a new tab so I can get its address from there. Then I throw that address into a forum post and wrap the [img] tags around it, like so:

[img]https://pixpet.net/usersc/images/items/bdad2f5841e5704c.gif[/img]

On the other hand, if there's an item you don't have and you need a picture of it, you can go to the wiki (which is linked at the top of the forums, next to Credits) search for it if you know the name, and get the image address from there.

I hope that helps!