This is how you make an HTML game with photos/gifs. I wish more creators would take a page (multiple pages, actually) from this book right here.
a) There are clearly defined characters. Scenes with a character show you this character only (or, at least, if she isn't at least it's a "symbolic", faceless stand-in). I have seen multiple games, where you don't get a clear sense of characters. Either there's too many characters, too few pictures or just random pictures. In other words: The choice of pix and vids is essential here.
That means: Choose adult actresses, where you have enough "material", that you can show them in various scenes and situations. Also, don't forget that you need scenes, where they are all dressed and proper.
b) The escalation of XXX is not too fast. This is something that SO many creators and Akabur-wannabe's get wrong. You don't start with extreme stuff. You have to tease the player a little, making you work for even small things, before you go all-out-nasty.
c) Stats and game mechanics are clear. Too often, you get games, where you have some random numbers and no idea how to influnence them or even what they do. On the other side of the spectrum, there are games, where the mechanics are too simplistic, where the actual gameplay is actually just a chore to get over with to get to the good stuff.
d) Decent writing. Noone expects Shakespearean drama from a porn game, but you should at least know enough English to express some basic emotions and maybe even have some skill at writing scenes. VSN and HTML games involve a lot of reading, so you better make sure, that the writing is competent enough for people to actually enjoy clicking through endless pages of text. At the same time, avoid walls of text and make the dialogue snappy and to the point. I know, it sounds complicated, but if you can't do that, maybe creating text-heavy games is not for you.
Keep up the good work, Westane! Sorry for the rant.