I am working on a project which has to store tens and thousands of images on a server and let the users access them. I need the most efficient method to store these images and to retrieve them. Also, I need information about which technology I should opt. I haven't started the project yet. So, I am thinking between PHP w/ CodeIgniter and Ruby on Rails.
The site hosts many images that are uploaded only by the Authors of the content, and not by the users.
When you store images, it is beneficial to generate a checksums for them so you can check if images are duplicated and store only one instance.
Organization of folders is important as file system seeks images just like a relative database and many images in one folder would be a problem. When you have already implemented checksumming, you could easily create a directory structure from checksum, for example first two characters of MD5 would be 1st level of subfolders and two next characters would be a second level. The remaining characters would be the image file itself. If you want to allow duplicates, after checksum you can add a suffix for image name.
Regarding storage - you should have a separate static content server with fast and specialized web server like nginx or lighttpd to forget about performance issues. HDD should set up as a a RAID array.
EDIT: For your web project requirements, just store information about images in the database, used framework is not relevant to image storage itself.
If you use PHP, you could always use some of the software available for image storage and manipulation, like Gallery, or phpThumb, or at least, have a look at their code so you don't have to reinvent the wheel. Having said that, I am sure there must exist similar libraries for ruby.
Look a bit around, this is very common question. But these links might help you
You may want to research more
Edit#1: One suggestion, just don't store your images in database.
