There is no specific or optimal width. Let me explain a little 🙂
Content Width: 750px
Case 1: Uploaded Image width is 750px or less than 750px.
Result: Image will be displayed without responsive resizing in browser.
Case 2: Uploaded Image width is greater than 750px
Result: Now the image will be responsive resized in the browser. Resizing will maintain aspect ratio. Now you can learn the Aspect Ratio by uploading images with different widths and heights. It is just like resizing in Photoshop. If the height is two big than the result will be different and if the height is small result will be different.
To avoid such confusion, we use good portrait or landscape images with the optimum width and height by keeping the file size in mind.
Your observation is sharp, Yes image with 1200px width with the standard height is a good option.
I hope it will help you.