Great question and one thats definitely worth a deeper explore.
First thing to keep in mind is that the answer might be different depending on the category. There's also a difference in clicking an ad and in the ad generating enough interest to get one to reply after they have clicked.
For me personally if its an ad for a house or a car and there's no picture I'm less likely to be interested enough to reply if there's no picture and there happen to be other competing ads that are just as interesting. If it's an ad for a house that by the description sounds exactly what I'm looking however for I'm probably interested enough to at least do a drive-by and then phone the person to arrange a time to have a look... but without the picture it's harder to get my attention. Rental properties on the other hand you'll not see pictures of very often and the feedback I get from landlords is that they get so many replies that they often have to delete their ad once they've found their tenant to stop the replies!
Job ads also have little need for a picture in most cases, thought employers will often put a logo.
Going deeper, some stats that surprised me.
I ran some numbers on all ads currently in the database. Ads without pictures had an average of 445 views. Ads *with* pics averaged 342 views meaning that pic-less ads are actually viewed more. This is a bit deceptive though since the Jobs category is so extremely popular and the amount of views on job ads bumped the average view total up for the site as a whole. Excluding job ads postings without pictures averaged 288 views, or about 16% less.
Because I like running numbers
.... A better comparison still might be to compare numbers within categories. For example, in the Used Cars category ads without pictures averaged 280 views, with pictures averaged 399. Ads in Agricultural Merchandise averaged 345 views without pictures, 499 views with pictures. Furniture had 228 views on average without pics, 362 with. For Baby and Child Items the difference was quite minimal... 160 without, 181 with pics.
Hope that helps!
Edited by Adam, 2016-06-03 08:42:41