Most web pages are built with a resolution width of 1,920 pixels. If you right click on an image, and check the image info, it will give you the sizing. If it’s a picture you want to go all the way across the page, it should be about 2,000 pixels. If the picture covers about half the page, around 1,000 pixels wide will work. For a 1/4 size image, then 500 pixels will suffice.
When in doubt, send us a larger image, and we can size it down.