i upgraded my 2008 i mac 10.7.5 to elcapitan and i photo no longer functions

In the early part of 2015 Apple stopped supporting iPhoto and its paid for Photographic app aperture and introduced Photos, this was part of Mac OS X Yosemite 10.10.3.

However do not be dismayed hopefully you can get iPhoto back working if you want. Read this post very carefully, Can't Update iPhoto because it is not available in the App Store it all depends on what version of iPhoto you running prior to upgrading to El Capitan.

The instructions listed in the link mention Yosemite but the instructions will work for El Capitan.

If you open Photos you will asked if you want to migrate your iPhoto library, you can choose to do this but some believe that Photos is not as good as iPhoto and migrating your iPhoto library to Photos will render it unusable in iPhoto.

Up to you really. As you are on El Capitan there will be no improvements to Photos as El C was replaced by Sierra and now High Sierra. iPhoto although no longer supported continues to work in Sierra and High Sierra. Photos has improved in High Sierra.

When you download new images to your mac put them in a folder and then you can import them into each application so you can see the differences between the two apps, you can then see which one you want to continue to use on your mac with El Capitan.

If at some later date you want to buy another mac you may find that iPhoto will no longer function on a newer mac with a newer OS, but no one can say that for definite. Photos will continue to be supported as it is still fairly new and should improve.