I just did this about 15 seconds before you asked the questions, what are the odds.
You will need to decide which image you want to show and get that Record ID# summarized to the parent record.
So if you have some qualifying field on the Documents table that says what type of file is attached, it will be much easier.
Say you have document type of "Photo"; Create a summary field where you are summarizing the ""Minimum" Record ID#, with the condition that "Doc type" is equal to "Photo".
Call it something like [Minimum Photo Record ID#]
Then use this formula in your formula text field on the Projects table.
"<img height=\"80\" src=\"" & URLRoot() & "up/" & [_DBID_DOCUMENT_REPOSITORY] & "/a/r" & [Minimum Photo Record ID#] & "/e6/v0\" />"