If you're just wanting to show a pic hosted elsewhere, just put image tags around the url for that image (which you can get by right clicking on most images on the web). Like below but using square brackets instead of parentheses:
(img)http://www.placeholderimage.com(/img)
Here's the url I will use below:
http://www.golden-gate-park.com/wp-content/uploads/2011/03/conservatory_of_flowers3.jpg
Posting it again surrounded by image tags using square brackets instead of parentheses:
But if the image is on your
PC the easiest way is to upload it to the Dakka Gallery using the Gallery link at the top of the page. After uploading, you can find your gallery images by clicking on your username (see there is a link to my gallery pictures under my username to the left here, yours will also have that after you upload some).
Once you've uploaded some pics to your gallery on Dakka, you can click on any of those images and go to a page with just that image. On that page there are easy links on the right hand side that you can copy and paste into a forum post... And they'll show up in your post preview if you want to test it out