Old Mac upgrade

My 20in iMac was purchased in April 2008, a mid 2007 model, with 2.4Ghz Intel Core 2 Duo processor and 2GB memory, and is currently running 10.9 Mavericks. In the past, I normally installed every other major upgrade to the OS, but for some reason I failed to install El Capitan. Probably as over the last few years I have made much more use of my iPad.

I have been attempting to reprint photographs taken 40/50 years ago, the colours on the originals have faded and they are so much smaller. I am disappointed with the performance of iPhoto, the print colours are far worse than the screen image. I would like to try Photos for Mac but need El Capitan.

On Amazon, I found a retailer of an upgrade supplied via a USB drive but it will not install. I have read that my iMac will only accept DVD upgrades. Are these available?

Thanking you for any help you can provide.

Poor colour reproduction in photographs is more likely down to your printing options than the software you’re running.