Most (?) forums don't allow you do directly type in < > html code (prevents xxs and sql injection)...instead they came up with a system which uses [ ] 's ..yes...not a huge difference, I know, but that's how the html interpreter is designed. Anyway, somewhere on a page you should see "BB Code"(ie. not <html> Code) ..click on it and it shows you some of the [ ] tags you can use. Most of them are the same as html, just with [ ]'s instead of < > ' s. ie. [strike]this code not supported yet[/strike] isntead of <strike>..</strike>.
[ img ] https://www.mysite.com/myimage.png [ /img ] < -- no quotes needed, take out extra spaces ofc.