I'm using Photobucket but there are others.

I use DropBox which is a service that gives you 2 free gigabytes of public storeage. It also gives you folder-like access to these files from you computer. Files in the Public folder of your DropBox folder are publicly accessible. You just right-click over any of them and select "Copy Public Link" and that will grab the necessary public URL string that you need. In your GardenWeb post you insert the following:

(a href="URL")Label(/a)

or

(img src="URL" /)

You would need to use angled brackets () instead of the parenthesis I used above. I did that just so it would show up correctly.
Where the above examples have URL you would paste in the public URL from DropBox or any other source, keeping the double quotes.
The first example with (a href) puts in a link instead of an actual image. This is useful since people can choose if they want to download the image or not. In this case your replace "Label" with the appropriate link text.
The second example (img) inserts the actual image right onto your posting.