Actually, it's quite easy to determine whether extra RAM would provide a noticeable improvement to your computer. If you run the application "Activity Monitor" (normally found in the "Utilities" folder within the "Applications" folder), and select the "Memory" tab, you'll be presented with a list of apps in order of their memory usage, along with some statistical information at the bottom of the window. The "Memory Pressure" graph provides a very general estimation of how well memory is being managed, but the two bottom values, "Swap Used" and "Compressed" will give you exact values for how much benefit you could derive with more RAM.
First, when OSX runs out of physical space in RAM, it tries to compress rarely-used data. This is a tradeoff of space for time; it allows you to fit more data into RAM, but requires CPU power to uncompress that data when it needs to be accessed. It's a cool trick, but only goes so far.

You can only compress data so much; after that, the OS will start to "swap" regions of memory out to the long-term storage (HD/SSD). This requires even more time than compression does.
If both "Compressed" and "Swap Used" values are 0, you'll get no benefit at all from more RAM. If you've got a fairly high "Compressed" value, you'll probably get some benefit from more RAM (as long as you get enough to avoid any compression). And if you've got any "Swap Used" value, you'll probably see significant improvements in speed from any increase in RAM.
(And yeah, you'll probably want to check the Activity Monitor when you are putting the most stress on your machine, such as the photo processing you were mentioning.)
BTW, let me go ahead and mention my (highly unpopular) point of view on SSDs. The SSD serves the same purpose as a hard drive -- it is used for persistent data storage. For most people, it is mainly used when loading data into the machine; so, at boot-up time (loading the operating system), and when starting applications (loading the app). The SSD significantly improves these two operations. It does _not_ significantly improve anything else. For myself, I tend to leave my computers running 24/7 (so I'm not constantly turning them on and off), and I tend to keep my most important apps running (so I'm not constantly opening and closing and reopening them). As such, I don't really see much value myself in the SSD; more RAM provides a better boost in speed for me...