You can upload your pictures to flickr.com. Other photo sharing sites will work as well, but I am not familiar with them. Once you've done that, pull up your flickr photo stream and click the picture you want to use. Once it loads, you will probably have to right click it and click large where it says "view all sizes". This will pull it up again, but this time when you right click it, it should give you the option to "copy image location." Then when you write your post to garden web, you can type [img src=" "] (Replace the [ and ] with ) Inside the quotes, you will right click and paste the image location which you copied from flickr. Make sure to hit preview before submitting your post so you can verify the picture's there.
Hope this helps.