Here is an outline of one way that works using Photobucket.
Open a free account on a photo hosting site like Photobucket.
Upload photos to Photobucket from your computer.
Go to your Library or Your Bucket on Photobucket and click on the photo you want to post here.
Use the Photobucket edit tool (paintbrush tool, floats over upper right hand corner of image, gets you to editing, choose resize) to resize your photo so it displays properly on the forum.
A width of 720 is a good starting point for a horizontal image, 540 for vertical. Click Apply.
You could also do other editing at this point, like Sharpen.
Click the little box for Replace Original (under Save) and Save your changes.
Note the http: links to the photo provided on the rhs of the screen.
Start a post on here and click the Mona Lisa image icon in the forum editing board or type
[ img ] [ /img ]
(but without the blank spaces inside the brackets).
Go to Photobucket and click on the Direct link for your photo. It should flash yellow and say "Copied".
Paste that copied link between the [ img ] [ /img ] you created above [ img ]http://i567... [ /img ] . Add whatever text outside the image you want. Click "Preview" in the editing pane and see if it displays what you wanted.
