You would have to right-click on the thumbnail, choose "View Image" (in Firefox) or go to "Properties" (in Internet Explorer) and get the URL that that image, and then paste it here between [img] and [/img] tags.
Most people post the "medium" 640 x 480 size here. If you view an image in the gallery that's bigger than 640 x 480, you'll find it generates a block of [img] code. For instance, if I go to Chilhuacle's photo here:
http://www.feldoncentral.com/garden/...0037b.jpg.html
There's a box of code at the bottom that, when pasted here on Tomatoville, looks like this:
Click for Larger Size
That "medium" sized image is 60kb which takes about 10-12 seconds on dialup.