Do a Google (or YouTube) search on what the channel art dimensions is that recommended. Try to use a photo that is large and crop it down to the size you want. The object/subject in the photo should not take up most of the photo so that you can crop it to the right size without having to lose the object you want to remain in the final result, then test the image by uploading it and see the preview on different platforms, usually there will some additional cropping on mobile platforms so try not to have too much text and place the text more towards the center of the image so that it is visible on all platforms.
Honestly I find it too much of a hassle for me to do all the above (for text) so I just use a simple photo but cropped in such a way that it maintains all the key images on all platforms, at least that's what I feel.