You use picasa to make a copy of the image that has reduced size -- I use a couple of programs but not Picasa, and in both I usually find that if the longer side is limited to 850 pixels, then everything is fine. I suspect you would want to use an export function in picasa, but I am not completely sure about that -- in any case, you need to know exactly where on your PC this reduced size image is located.
Then if you are talking about Birdforum gallery, go into that tab, click the icon for uploading, use the browse button to get to your reduced size image, type in the title of the photo, and click upload; fill in the remaining information wanted, and click submit, and everything should be there.
If you would like to get the scientific name for a bird (appreciated), open Opus in a different tab, search for the bird and copy the scientific name from there into the upload form.
hope this helps
Niels