At the least you'd need a field something like:
{IF{MERGEFIELD "Apartment_Record_Type__c"}= "Third Party Unit" {MERGEFIELD "IMAGE:Bedroom_Generic_URL:W=250"} {MERGEFIELD"IMAGE:Slideshow1_URL:W=250"}}

where the { & } are all field braces, not typed braces.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]