I have a 2008 24" Imac (1 terabyte HD) running OSX El Capitan - can I upgrade to a higher OS?

I'm afraid, El Capitan is the highest macOS version you can install, these would be the specifications for Sierra:

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MacBook introduced in late 2009 or later

MacBook Air introduced in late 2010 or later

MacBook Pro introduced in mid 2010 or later

Mac mini introduced in mid 2010 or later

iMac introduced in late 2009 or later

Mac Pro introduced in mid 2010 or later

...

copied from: How to upgrade to macOS Sierra - Apple Support