I think you should build excellent site that is user friendly (user friendliness includes labelling images for blind or partially sighted people) and build quality links to your website, all the above are also "search engine marketing techniques" which will stand the test of time.
The technique that will go out of fashion are short terms one designed to just promote a poor site artificially.

Originally Posted by Cleo8

My question is, from what you all know, is it really worth the time to label images

I can only talk about my own experience, but labeling my images made it possible to find all my pages by Google.

I have about 500 books I discuss on my site and every image has been indexed by Google : www.boekensite.net - Google Image Search

As a result, many of my visitors came to my site by searching on images or pictures...

For Googlebot the ALT tag is the only way to know what a graphic file is all about. So, I dont think it is or will ever be a waste of time to lavel them.

If by 'label images' you mean using alt attribute with img tag continue reading on, otherwise disregard...

I don't see why 'labelling images' could ever be treated by Search engines as not important. The only thing you should remember is not stuff your labels (alt attributes) with too many keywords. For example

img alt="Books, buy books, cheap books, more books, adventure books etc" could be easily detected as inappropriate behaviour

..

Originally Posted by maxseo

img alt="Books, buy books, cheap books, more books, adventure books etc" could be easily detected as inappropriate behaviour

I suppose you're talking about my example...according to the use of "books" .
But yes indeed, you are right.
In my case, each image has another tag and is always build the same way : Authorname followed by booktitle. So, this way there are no duplicates and... it works well

The alt attribute is meant to describe the content OF THE IMAGE. so how can the content of the image be authorname booktitle? It should say cover of booktitle written by authorname. Now that would be correct use of the alt, get it? ALT = Alternative, a text representation of the image.

And to answer your questions google stopped paying much attention to H1 H2 etc a long time ago. It is now more about semantic relevance.

Originally Posted by OldWelshGuy

The alt attribute is meant to describe the content OF THE IMAGE. so how can the content of the image be authorname booktitle? It should say cover of booktitle written by authorname. Now that would be correct use of the alt, get it? ALT = Alternative, a text representation of the image.

Maybe my english is not good enough...but what you are saying is exactly what I do.
Otherwise...just take a look at the covers on this page : Eric's Boeken Site - Boek Opstanding van Greg Iles and tell me if it is so different of what you think I was explaining...

Yes that is about right. What you have to remember is that these are often read in a screen reader, so would they read right? If the answer is YES, then fine. I would be tempted to add the word cover to the titles. so it read
'Front cover of booktitle by author' but that is more usability than SEO. although it will help with both PROVIDING the image is a link, if the image is not a link, then the alt is worthless for SEO.