Did you ensure all the spaces indicated in my post exist in your field construction? Omit any and it will likely fail in exactly the way you've described.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Hi, I spoke to Conga support and was able to use
{IF "{MERGEFIELD _RECORDTYPE}" = "Third Party Unit" "{MERGEFIELD IMAGE:Bedroom_Generic_URL:W=250}" "{MERGEFIELD IMAGE:Slideshow1_URL:W=250}"

I also had the wrong label. I was using the API name Apartment_Record_Type__c when I needed to use the Conga label of _RecordType. Thanks for your help.