FWIW, the current picture posting method and restrictions for
this forum are explained in
this FAQ.
If you check the "Properties" of each thumbnail (which represents a larger image), you'll see there's a difference and further checking reveals each time another copy is uploaded (the forum software doesn't "know" the difference--if any--between one image and another), it's more space redundantly used that must be supplied and maintained--disk space, database, backups, etc. Individually insignificant perhaps, but a scalable cost nonetheless.
Unfortunately (and perhaps for unstated reasons--there
are other considerations) the option is not offered here, but ordinarily if you have an image already hosted somewhere and openly viewable, most forums based on the vBulletin software (used here) provide an "Insert Image" option that inserts some code (see below) somewhat similar to a regular "link", which tells your browser to display an image and where that image is located.
<image src="http://icreek.com/stove/howto/5_480.jpg" alt="Insert Image Button example">
If enabled, instead of the above code, everyone's browser would display an image from any site as if it were part of the current site's page.