I am building a large site at the moment, the main content (visitor attraction) of which is unique images, so its basically a gallery. All the images have at least two words in their name and I am wondering whether to leave them SPACED as i have now or HYPHENATE them? This may be critical to the success of the site so I am hoping for some definitive answers if possible. Which would have more success with google searches?
In the alt tags or title tags, definitely use spaces between words. In the url, definitely use hyphens, as opposed to spaces or underscores. Strangely, Google counts the underscore as if it were another consonant or vowel. So Google sees the phrase in the url ...blue_picture... as one word "blue_picture", instead of two words "blue picture". Spaces in the url will frequently become "blue%20picture" and on some sites "blue+picture". Google treats hyphens as dividers between two words.
Add comment type text under every image to represent what the image all about I think will help. Plus emphasize them, use header tags or bold/italized.
I don't think hyphens or not particularly matter but if in doubt use them. A third of my hits from Google come for images and I do not use hyphens.
I believe the key to success with google images is more to do with the text before and after the image, and the alt text, than with the filename. Also filesize is important (in pixels not kb) - they need to be a decent size to rank. I just looked at some popular searches in images and for the ones I looked at there was as many underscores doing well as hyphens. But as for leaving spaces in the filename, I never tried that just because I don't like spaces in filenames, and none of the results I looked at had used spaces either, so I don't think it will help...but I can't be sure. personally I always use hyphens and I get a lot of traffic from google images.