What you need to do first is query the Metadata table for the particular Resource/Class you are looking for the PhotoCount column. These won't be 100% correct since I haven't ever used connectMLS, but you will get the idea.
Use this query to get the metadata for the table Property:
http://<connectMLS.rets.server.url>/rets/server/getmetadata?Type=METADATA-
TABLE&Format=COMPACT&Type=Property&Class=ResidentialProperty
<COLUMNS>
SystemName StandardName LongName DBName ShortName MaximumLength DataType
Precision Searchable
Interpretation Alignment UseSeparator EditMaskID LookupName MaxSelect Units
Index Minimum Maximum
Default Required SearchHelpID Unique
</COLUMNS>
One of the available columns above will more than likely have a PICCount or Count or PhotoCounty something related to count. If if doesn't then you will just have to process your returned results and count them yourself.
I hope this helps. It has taken me a little while to wrap my head around RETS.
What particular MLS are you writing queries for?