The forum already supports uploading big photos, you just can't insert them in a post that wide because the page container isn't that wide. This is 1920x1080 as large as it will allow:
I'd be all for increasing the maximum width of the forum container because it only takes up about half of my UHD screen, I'm not sure we need massive images though. They just take up too much space. Most of the images I post are hosted on my own web space so I have control over them, I just link to the full image with a 1024px thumbnail. This is the code I post, made from the image URL using a Notepad++ macro:
Which produces this:
If you still want to attach images, delete it once it appers in the post, then click on the attachment below the post and the full-sized image will open. You can then populate my code with the internal URL. This is the uploaded image from the top, made to fit with a 1024px thumbnail.
I use 350px thumbnails when I have lots of images, made from another macro.