Hi Cressst,
There's a very good website which has all sorts of useful information for owners of holiday homes, called
Lay My Hat. You'll see there's a page about the best sites to advertise on and there's also a forum for owners. I found it very useful when I was starting out with internet advertising but I haven't looked at it for a long time - must do so in the New Year when all the rush and panic have subsided!!
I think it's generally true that you get the best results from paying websites. I'm not sure where your holiday cottage is, and if it's in the UK there may be more suitable sites for you to advertise on, but we find Holiday Lets and Holiday Lettings to be two of the best for results.
It is a very good idea to put the kind of search words into Google that you think your potential guests might use, and then see which sites come up on the first page or two. Unfortunately there's no substitute for hours of research, going into all these sites and searching for properties similar to yours to get a feel for which website(s) would be best for you. If you want to try a few free ones first, just put "free advertising for holiday homes" or something like that into Google, pour yourself a nice long drink and get down to your research! The more adverts you look at for homes similar to yours, the more idea you will get of what to put into your own advert - and it really is true that photos are the best sales tools.
Good luck!