2 ways. Either click on the Cat Photos link on the top where you can upload picture directly to this site, or check out the "sticky" in the Pictures section of the forum. You need to use a photo hosting website to include pictures in forum posts.
Unfortunately there isn't such a way. You must have your pictures hosted on a website in order to link them here. You could do as Richo said - upload them to the cat photos section of this forum and then link them to the thread you wish to post them in