Images should to be resized to the correct size and compressed when uploading to help with page speed which in turns affect the user experience and SEO. If you are familiar with Photoshop, you can use this to resize and compress images - JPGs are normally the smallest in size. Other options include:

Compress

• tinypng.com
• This free tool is simple and powerful; upload your photos, then download the compressed files.

Resize

• Go to bulkresizephotos.com.
• Select photos to upload.
• Go to longest size and set longest size to 1500.
• Select expert mode and choose file type JPEG & quality 80%.
  • If you want to maintain the transparency of an image (i.e. no background) - resize these images separately and save them as file type PNG (i.e. not jpeg).
• Click "Start Resizing".
• Once complete, the resized photos will be zipped in your downloads folder.
• Unzip the folder to access and upload the resized photos.
• Demo - And here is a quick demo video on how to do it.