If you were just a hired session musician, you wouldn’t be in the mags, posters, etc. at all. Session musicians are folks like the professional singers that the Beatles hired, who used their highly trained vocal skills to sing the “oompa loompa stick it in your jumper” (what Paul McCartney says they sang… some people think there were some words about smoking pot thrown in) type stuff in the background. You come in, you play for a day or two, you get your paycheck, and then are never seen again. You also don’t get paid ever again.
Some bands have “tour only” members. Being an avid Genesis fan, this is the group that comes to my mind. On all of the posters, magazines, and such, there are only three members of Genesis, and I believe they do most of the studio work as well. In the studio, Phil Collins sang and played drums. Mike Rutherford played the bass and guitars. When they get on stage, they can only do one or the other, so while on tour Chester Thompson played drums and Daryl Stuermer played the guitar. Chester and Daryl were never in the magazine interviews and such. Only the three main members of Genesis (Phil Collins, Mike Rutherford, and Tony Banks) got the magazine and tv interviews.
You, being a hired synth player, would get a similar treatment. Since the band isn’t an established band like Genesis, the contract with the record company sucks. They are all working as slave labor. The big fat guy with the cigar is collecting all the real money, not the band. You are probably complaining to your girlfriend (that you just met yesterday, and you’ll have a new one tomorrow) that the band is filling stadiums and they aren’t paying you squat. You are probably paid a small percentage of the sales that come in to the stadium, and it’s likely to be a very small percentage. Once the tour is over, you unplug your synth and Crowded Flock of Kajagoogoos from Berlin is left behind forever. You don’t get any more money from them, and VH-1 probably won’t even call you 20 years later when they do a special on one hit wonders, unless of course you can tell some really wild lies about what the band members did in the hotel rooms.
On the other hand, Chester and Daryl, who do essentially the same thing you do, make much more money at it because they are members of an established band (which means the original slavery contract has expired and they’ve negotiated a nice sweet one now), and Chester and Daryl both have a reputation which allows them to demand more money. I don’t know anything about the inner workings of Genesis and have no idea what is in their contracts, but I suspect Chester and Daryl don’t get anything from the albums, with the possible exception of the live albums like “Three sides live” and the “Mama” tour that was released on video.
A friend of mine from High School was the drummer for Joan Jett for a short period of time. You can see him in the movie “light of day” (I think he even has one line where he actually speaks). He got to live the life of a rock and roll star. He had a lot of money, for someone who was just a poor high school kid a few months earlier, but he wasn’t a millionaire by any stretch of the imagination, and of course he had more girlfriends than he knew what to do with. Joan was the one making the big bucks though. PJ was actually a member of the Blackhearts (Joan Jett’s band) for a while so I don’t know if that really qualifies as what you are calling a “session musician,” but it does seem to be pretty close to what you are describing.