1) .xcf is the filetype GIMP uses (I also don't know what those letters mean), so you won't want to turn it into a PDF as part of your process. In fact, I keep a saved copy of the blank templates as .xcf files (and some additional modifications, like grouping all the base wolf layers into a layer group and adding a flat color background layer) for easy access for anytime I want to start a new CA.
I have never exported as a PDF before, but I did some browsing and it sounds like you may be able to (but I can't tell if it needs a plug-in or not). A note of caution is that text-data is not preserved normally when exporting to PDF (the image wouldn't be affected... so I'm guessing this is for like screen-readers or something? From what I gleaned it likely wouldn't be relevant for anything CA related).
2) To upload an image onto the forum, you have to have the image hosted somewhere external. Once your image is hosted somewhere, right click on the image and select Copy Image Address. Then you can just click the "Picture" icon in the forum reply bar and insert the link to the image, and it should show up just fine.

I've always used imgur because it's free and doesn't require an account to host your images, and I haven't ever had any issues with links breaking. There was a recent change imgur no longer works if you're in the UK, so our UK friends unfortunately wouldn't be able to see them if you do use imgur. I am a creature of habit, though, and haven't gotten around to finding a new hosting place yet so happy to take easy recommendations but if you use imgur it should work for everyone except the UK folk and you won't be alone in doing so for the time being.
Also just a note that sometimes I have to Copy Image Address multiple times when using imgur. Idk if that happens with images hosted elsewhere or not, but sometimes copying the image address copies something like blob:http://example.com/a1b2c3d4-e5f6-7890-1234-567890abcdef, which is apparently referencing an image from your browser's memory rather than the actual site. If that happens just copy again a couple times until it gives you a normal looking image address like you see pictured in the screenshot above.