I am pretty sure that you need to check howrse on doing that because you are not allowed to post any links that are not other than howrse.
[url= URL GOES HERE][img] IMAGE CODE GOES HERE [/img][/url]
Depends on how the forum software was initially setup by the site administrator. If enabled, there may be a 'submit attachments' button below the text entry box ... then one would enter the file name...
Use the IMG Tag.
Typically you'll see a 'post thread' link. Though this varies from forum to forum. If you provide my with a link I can see if I can find it for you.