Well choosing sponsors for galleries is actually one of the hardest parts, once you have the gallery design down, thats the easy part.
Second is getting listed. Where do you plan on submitting, all tgps have different rules. As for me I stick to a few free listings and I also buy listings on some.
As I said it matters where you plan on submitting man, lets say for instance thehun.net he has rules regarding recips as well as others do.
back to sponsors... the hun for instance wont accept a gallery with certain sponsors on them because they have exit or pop consoles which "annoy his surfers" well so you obviously cant use THOSe sponsors, dont ask me which ones they are as I dont know or care.
Why dont I care? Well if you make a FPA, or link the links on your gallery page to a SECOND page THAt page can have links to WHATEVER sponsor you want, as long as its not directly effecting his surfers with pop ups, after they clickthrough, the surfer is yours, but while on the gallery it is still the Huns surfer.
So choosing a sponsor depends on MANY MANY things, such as what sponsors would convert on some tgps more than others, obviously you cant list a Mature gallery on a teen tgp, so you gotta think along those lines.
