Is it the one that's in your blog? If you want to use the full-size image, click on it and you see the picture on it's own. Copy the url from the address bar or right-click on it and select "copy image location" or similar (make sure it's the location/address/url you copy, not the actual image). If you want to use the small version that shows in your blog, right-click on that and copy the location. Now you can paste the address wherever you need it.

Thanks Anika - I cheated in the end and asked my web guy to do it 

But I already explained it! If anyone's having trouble with the explanations and wants more help I can see what I can do, but this is really not advanced stuff, it's just using one of the functions of your web browser or wordpress.

Ok, I'm going to try to make this more simple because I'm sure there are other people here who don't know how to do it. The wordpress bit only applies if you have a wordpress blog, but most people can copy the url of a picture on their website.

Firstly, you need to know about copy and paste. If you don't you really should: it's a very simple function that all computers have to make things easier for you. See this page.

Now, find the picture you want on your website and follow the instructions here for your browser. (If you have no idea what browser you're using it's probably Internet Explorer.)

Now the address of your image is on your clipboard, which means you can paste it anywhere. If you want to make it appear as an image in a forum you can often do so by putting it between two IMG tags, like this.
[IMG]http://yourwebsite.com/yourpicture.jpg[/IMG]
That's it.

If you tried to get the address of your image and everything looked wrong it's probably because you have a Flash site. Getting the address of an image from one of those takes more steps and I'm not going to go into that. Make sure you're viewing the html version of your site. If there isn't an option to do that on your home page you can see it by disabling Flash on your browser (you can enable it again straight after). If your website doesn't work with Flash disabled you have a bad site and I hope you're not paying for it.