Email servers limit the size of attachments that are sent. Most servers limit the size of an email to about 20Mb.
If your computer is a Mac you can airdrop the photo to it, or even send an iMessage to it. The iMessage is limited to 100Mb.
If you computer is not a Mac, the best way is to use a third party service like DropBox, Google Photos, or even iCloud Photos. Just send the photo to the service using you iPhone, then on your computer, you can import the photo.
Another option is to make the picture smaller before you send it.
Let me know if you need help with any of these options.