How can I burn this slideshow onto a disc so I can play in my DVD player and watch on tv

You need software that will encode your video so it can be read by any DVD player. Since Apple abandoned disc burning, it's difficult to find burning software.

Toast is an excellent app to burn videos (and other things); there is also a free app called "Burn" - very basic, but it works. iDVD was (still is) the best because it has very cool introductory menu and transition options; it will still run on El Capitan but is a bit unreliable - I have a Macbook Pro which can boot from Mavericks, El Capitan, and Sierra and just created a DVD using El Capitan. You can still buy it - it is included in the iLife 11 suite which is occasionally available on places such as ebay or amazon, but I do not know if it will install on newer OS versions (mine has been installed for years). So it'd be at your own risk (but it is very inexpensive).

Here is a user tip on burning DVDs:

How to create a video-DVD? (Macs without iDVD)

Amazon.com: iLife '11 [OLD VERSION]

I think you have recorded a data DVD, which is like a read-only hard drive a computer can read. Some newer DVD players might be able to view photos and video files on this kind of DVD, but not older ones. You need to use the /Applications/iDVD.app to record a DVD in the correct format for a DVD player to watch on a TV.

My mistake, iDVD came with earlier versions of Mac OS, and I guess has been retired with El Capitan, around the time that OS was released some new macs were not shipping with built-in DVD drives anymore, although external USB drives are still available. If you had an older version OS on your mac when you upgraded to El Capitan you might still have iDVD in your /Applications folder, but a clean installation would have deleted it.

So, you can buy other software to create a video DVD, like Toast Titanium.

http://www.roxio.com/enu/products/toast/titanium/

An old box set of iLife 11 includes iDVD

https://www.amazon.com/Apple-MC623Z-A-iLife-VERSION/dp/B003XKRZES

PS - you can test the video DVD on your iMac by using /Applications/DVD Player.app, if it recognizes the DVD you created and plays it, then most TV DVD players will also play it.

In your first post you said you burned a DVD, so I guess you already have a DVD read/write drive either built-in your computer or external to your computer. Toast Titanium is a software package that helps you burn different types of optical media, ordinary data discs, audio CDs that will play in a CD player, and movie DVDs that will play in a standard set-top DVD player connected to a TV.