Heck, I shoot my pics at 4000x3000, dump them to my PC, cut out the 'focus' of what I want via the free version of Paint Shop Pro, re-sample that image to 1280 pixels wide retaining aspect ratio, UL them to my web server and reference them via image tags in posts.
If the web site don't re-size pics for initial size like this forum does but my web don't expect in a certain forum, I re-sample to 750 pixels wide as that is comfortable size to view 'as is' for most.
Here is an example.
The first is a 1280 wide pic, the second is a 750 wide pic.
They look the same size for initial forum display, but only the first one gets displayed full size larger on clicking and the second is already the size the forum likes.