If you are trying to upload large images to WordPress, you may get an error message letting you know that the image file sizes are too large. It is also good practice to make sure your images are optimised for the web, so they are a reasonable file size and do not place a strain on your visitors' data plan!
You can optimise a batch images easily so that they upload and download quickly, without taking up all your disk space. Please see this page for more information.
If you just want to resize a few images, you can also use this online tool.