It depends on the editor you're using.
You need to resize it so that it's not too wide (some forum members may be using lower resolution displays and would need to scroll left and right to see anything too wide).
I'd go with around 720 pixels wide (but up to 800 is probably acceptable for most monitors anymore).
You also need to make sure the file size isn't too big. The max you can attach to a post is about 240Kb (give or take a little).
I try to keep mine at 200KB or less. If after downsizing to a smaller image size (width x height), if the file size is too large when you save it, you'll need to use more JPEG compression.
Most image editors have a JPEG Quality slider when you use the "Save As" option and select jpeg as the file type. Lower Quality = Higher Compression = Smaller File Size. Play with the compression (quality) amount until your saved file size is around 200KB or less and you won't have any problems attaching an image (unless it's too large from a width x height perspective).