What the LinkedIn image resizer does
The resizer crops an image to a LinkedIn size and saves it at exactly that size. Pick a preset, drag the image to position it, and use the zoom slider to tighten the frame. The crop box always keeps the preset's shape, so the file you download matches it.
It's free to use and works in your browser, without an account. On a keyboard, the arrow keys nudge the image and plus or minus changes the zoom.