i find it easiest to use a third party site like imgur.com. upload your picture there. click on the picture on imgur and they will provide multiple types of links to the image. the one i use is listed as BBcode. copy and paste and you are done.
now for help. we could use a bit more info like what fish are in the tank, water test results, how exactly is the tail hurt? is it torn fins, a red sore, a white spot? as much info you can give along with pictures will help.