I have iphone on my phone and I downloaded it onto my computer. Why can I not access my photos from my phone on my computer?

I'm assuming what you did was back up your iPhone to your computer? that is not equivalent to 'downloading' your phone onto your computer. An iPhone backup is meant to serve as a back up in the event the your phone dies or you forget your passcode. it does not result in something that you can browse thru the contents of

About backups for iPhone, iPad, and iPod touch - Apple Support

If you want to transfer and access your photos, review this link

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support