I quote the guidelines:
"If you'd like to post an image to the forum (we hope you will), it needs to follow the guidelines below.
Pixel dimensions:
For horizontal images, no more than 1200 pixels across (let the number of vertical pixels fall where they may). The forum is locked in at 1400 pixels wide so this eliminates side to side scrolling.
For verticals, you can technically go 1200 pixels wide as well, but we recommend 800 wide - it just seems to look nicer and helps keep you from approaching the last limit - size.
File Size:
The file size limit is 1MB - usually far more than enough when using the recommended pixel dimensions above. (We have to limit size or our server's hard drive would become overwhelmed in short order)."
Resize in Lightroom, Photoshop, or program of your choice.