Using the img tags is easy, well it's easy on a computer with a keyboard, same goes for most of the other tags.
Simply select the image icon below the bold button, this will insert the image tags, now you need enter image url between the tags which is the fiddly part on a phone or tablet because it usually ends up selecting part of the image tags. Or simply type [img] then paste your url then close it off with [forward slash chr img]
On most forums you get a little pop up which allows you to easily paste the link then creates and inserts the img tags correctly.