I like type (e.g. movie, political) followed by country/foreign, follwoed by sub-genre (war, horror etc) I also like to be able to see old versus new for example, 1960 and below/up I think the best is to have as many options as possible, so you can sort your material, for example, posters under $100, 1 sheets, half sheets etc.
Re the image, i like to see as much info as possible without having to click further - so ideally moving your cursor over a thumbnail which provides an info bubble with price, dimensions etc.
Are ye building urself a poster database lad?