Since multiple requests can slow down the speed at which a site loads, I was thinking that in the case of a gallery, would one large image containing all the thumbnails be better than loading individual thumbnails?
The large image would then use PHP to “chop up” the thumbnails and place them in the relevant locations on the page.
My main concern is would this have a negative impact on SEO? Since Google would only see one large image file, instead of many smaller ones. Would a way around this be to set the src of all thumbnails to redirect to the script that handles the thumbnail generation, where the image file name refers to a particular set of coordinates for that image?
As a rule of the thumb; for buttons/icons/stufflikethat use image sprites (one large image combining all images, uses css to only show a part between specific coordinates), for ‘real’ content images, just use separate images.
This has several reasons; icons, buttons and so on are images that appear on sometimes every page of your site and often multiple times on the same page. So it is really useful to combine them, as ie. it is really inefficient to start a new http connection to download an icon of 1kb (or less), imagine what will happen if you use hundreds. Furthermore this type of images are not important at all for your seo rank, only for the look of your site (but google doesn’t care if your site is ugly as hell or beautiful as a princess)
But on the other hand, ‘content’ images, such as thumbnails, photo’s of your holiday or baseball tournament are often big enough to rule out the efficiency part. As you can see in the chrome developer tools or firebug the browser will start the download of all images simultaneously. So downloading one image is pretty much as fast as downloading a hundred. But if you combine a hundred images, the download will be slower, as you have to download a larger bit of data in one piece. In comparison; pushing 2 gallons of water trough one hose will take longer than pushing the same 2 gallons trough 10 hoses. (offcourse this metaphore has it’s holes, but it illustrates my point).
But more importantly; google reads out the img tags and uses the filename (src), the title and (less importantly) the alt attributes to determine how your image should relate to your seo rank. Images do have a relevant effect on your seo rank! But google also knows if it is the same image showing, or a different one, so a sprite wouldn’t help you here. A script, with parameters saying which part of the image has to be loaded wouldn’t help you at all, I believe if you think it over you can figure out why 😉
So don’t bother about merging the thumbnails and stuff like that. If you want to improve speed, move your attention to caching and speeding up transmission. Some really simple improvements can be implemented by using for example gzip compression (google .htaccess gzip for that), proper caching headers etc.