You can do two things
Use the direct link and type [img ] and [/img ] yourself.
Or use the link that starts with [img ] and ends with [/img ] anyhow
.
These sites give you that link so you won't have to type it yourself.
It's up to you whether you think that is practical.
All the forum asks is a 'regular' link (starting with http:// and ending with .jpg) to an image.
But the forum will have to know that you want to display an image and not the link, that's why you have to add [img ] to say 'this is where the image link starts' and [/img ] to say 'this is where the image link ends'.
Forums aren't all that intelligent, you'll have to tell them
.