If your image file is bigger than 5MB you won't be able to upload it to your website. The reason why there is a file size limit on images is to keep your website loading speed fast. A slow website appears lower in searches and we don't want that.
If you are trying to upload an image bigger than 5MB you'll see an error message like this in the Dashboard.
If you aren't able to choose the file to upload it then it's not a .jpeg or .png file. Again, this is to keep your website loading speed fast.
Need help with getting more images for your website? Read this article for tips on how to take great photos.