You can do this automatically by pasting the link to the image, then selecting it and clicking on the [Img] button in the Post a Reply area.
Please bear in mind that if the picture is large it will distort the thread and people like me will have a hard time reading all the fascinating posts. So, if the image is large just use the link to point to it.
For example, here is the URL for an image:
http://ani.studiodada.biz/snimki/art_ga ... dravec.jpg
Here is the URL with the [img]and[/img] tags around it
[img]%20http://ani.studiodada.biz/snimki/art_gallery/images/zdravec.jpg%20
[/img] -- of course there should be no spaces.
