I think there's a limit to the size of the file. You can try reducing the file size at the cost of image quality and or cropping the image or alternatively post it to imgur and use the paste url feature
Select options when clicking upload a file and pick what is shown in my image. May have to do 1 at a time if it keeps failing
webp is a good file format to compress images down to size. Yeah, smartphone cameras are so good now, the default file sizes are massive. You can use a free image tool like GIMP to load the image and then export it as a webp or other format.