I am guessing you are able to upload your photo to an image host (think of it like an online storage warehouse).
Now in order for your image to appear in a ClubSNAP post, you need to know the URL (address) of that specific image. You then put the address within the tags so the software knows you are trying to hotlink an image.
I thought the sticky instructions were quite clear to be honest.
But, before you can do this, please go to Welcome to Flickr - Photo Sharing, create an account and follow the flickr instructions to upload your images to flickr. Once you can see your image there you can follow the Clubsnap guide I posted above. Basically the same applies to other image hosting websites in case you do not wish to use flickr.
For you testing please use the respective subforum: Garbage & Testing