Upload your image to AI (i use claude) and give it the dimensions you need (1750 x 700), and ask it to resize it. If the original image is an irregular shape you can add 'buffer' space to the image in a similar colour, etc so that it maintains the correct aspect ratio.

I have a project set up in Claude with the dimensions for the profile and cover image. I just upload each client image and tell it what size I want, and presto, it creates the perfect size.