RE linking a photo to a library part:
How do you do that with ArchiCAD?
I've wanted to do precisely that with PDFs, photos for equipment, plants, etc.
Here is what I have seen some companies do --- they insert their family into a project (regular .rvt file) and then they put their text/image info onto a sheet in the project. Then they set the opening view to that sheet so that you see the info every time you open it. The file is transferred/stored "as is", and then when someone wants to put the family into his/her project, they just copy and paste it from that .rvt file to another, rather than by inserting the family. I have tried it and it works.
But of course that info does not show up in the family properties ... a URL would work for that, as was previously stated. I suppose (?) you could also include a path to a central database (in the family), if you are working in a large network? .... just a guess. ... Or a link to a cloud site where you have (or someone has) uploaded the data/files/images?
If I need to put a PDF into the drawing I convert it to Word or Excel copy the text, create a text in Revit, and paste it into a created text. A long work around. You might also try to paste the PDF into Paint, save it as a JPG, and insert the image.
That functionality would be very handy, but at this stage it isn't possible. You can however put web links to more information in the family's interface. eg link to a PDF on the company's website. - This is a technique currently used by BIMstop.