Staudey I'd recommend using a third-party image upload service
I've had good luck with Gwenview. Just open the image with Gwenview, and use its resizing function. That's a lot more convenient that using a cloud service. Gwenview even preserves the aspect ratio if you resize one dimension.
For posting to the forum, or attaching to an email, I usually select x=800 and let y fall wherever it may.