2013 iMac and use OS X Yosemite.

Since you have iPhoto running on 10.10.5, you can continue to use it through Mojave, but it won't open in Catalina. In any case, back up your data and check your applications for compatibility before upgrading.

zmclearan: Photos was introduced with 10.10.3, but support for iPhoto and other 32-bit software wasn't removed until Catalina.

(178908)

The model and year of your Mac will determine which macOS you can upgrade to,

read the following links. As you have a 2013 iMac from 2013 you can upgrade to any

of the OS's listed below.

When you mention iPhoto do you really mean the Photos app.

In Yosemite 10.10.3 Apple dropped support for iPhoto in favour of the new Photos app.

Although it is possible to still use iPhoto in newer OS's ( ask later if you want to know how)

certainly up to High Sierra,

I am not too sure about Mojave, but it will never work in Catalina.

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

How to upgrade to OS X El Capitan - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

Thank you, I had forgotten which version of macOS introduced the Photos application. In my brief research I found that Photos was not introduced with 10.10.0 which lead me to assume it was introduced with 10.11.

EDIT