When you're updating images on your website it's important to match the size of the image to the current image that you're replacing. An easy way to do this is by finding the image in your media library and checking the dimensions before sizing the new image. 
Step 1. From your wordpress dashboard open your media library
Step 2. Find the image you'd like to replace

Step 3. Select the image and on the right side you'll see information relating to the image
Step 4. Use the dimensions found to size the new image and maintain formatting.