I have a site in the travel niche, and i would say stick with text ads only. Image ads seem to be more google spam, branding by large advertiser and other wasted impressions.
Larger text ad allow for individual hotel who are willing to pay more per click, then a middle man services who only earn 10%.
Matter of fact i block all large middle man sites.
to a hotel in a city trying to earn a customer $.80 per click is reasonable with a hotel room that cost $160 a night on a 3-4 night stay.
While a middle man company only earn industry standard of 10% of the booking so they are not willing to bid as high
Online travel companies know online advertising better then individual hotels and how to squeeze as much as they can out of their dollars "i.e. less money for you", and how to best possible adwords competitors.
block your expedias, booking.com, bookit, kayak, etc and allow direct hotels, airlines, travel agents, reality company to get access to that space and you earning will increase big time.
remember more then price affect bidding and which ad get shown. an Expedia with million of click a day can out rank a medium hotel who's goal is 50 clicks a day even if they are bidding more "tested this myself, even bidding at $1.00 per click, brand name advertiser beat me out for top spots for less money on my own site"
also consider trying a affiliate product.
I have had a ton of success with a private branding solution provided by a company call ihsadvantage.
They give you a private branding hotel booking engine "that allows "Cnames" and even have a hotel page builder to help you get on site SEO for hotels. Expedia also offers a private branding and it by far converts more, but ihsadvantage give you control, you can set booking fees "even as high as $100 per booking if you want", and you get the email address of all people who booked through your private branding site, and the best part is they pay your cut of the booking fee each week "not each month, each week"!
instead of passing off your travel traffic to adsense capture it for you self. Find what ad appear the most, learn from them, them replace that adsense unit with text affiliate offers to your own private brand.
I had pages about a specific hotel, i created ad directly relating to that hotel in that adsense spot that deep linked to my private brand and instead of $.35 a click i made $50 a conversion "with a 5% conversion ratio", and that page is now ranked in Google directly "not high". I made over $1,000 off that one page alone this yeah vs a few dollars from Adsense in the past year LMAO.
Also note that google is trying hard to enter the travel market and is trying to get in the middle of visitors and your site looking for travel products. "Google Place for query that don't mention travel", Google Places hotel ads, etc.
Your earning will drop in the future unless you employ tactic to capture your customers and beat Google to them "or at least before they start search for travel on google", or make your self as a point of reference.
I use facebook fan page, capture email from hotel bookings and inquires, send out newletters, and enage the fan page.
I also created my own Places map with hotels listed linking to my private brand and provide people with a way to share. I create top hotel list for areas and encourage people to share, and i warn visitor about booking hotels through google advertising, alerting them that "the top bider, not best/closest hotel is shown in their results, and to warn people to check my list, and map before booking" AND it works.
Google, and other parties are trying to squeeze travel affiliates out of the way, to increase thier profits, and you are seeing a early sign of it.
damn this was a long post, sorry.
This advice probably won't work for all niches, but it's successful in travel and my niche.