One site in particular has over 4,000 images and grows at about 100 images per month. It's absolutely rock steady and I highly recommend it.
The only shortcoming is when you add images, there's no quick and dirty way to single out the new files over the existing files--I do a browser search.
Anyway, it's a great extension and it's free. BTW, I've donated to the author and plan to donate again because it's so useful.
I started off with the free version and liked it so much I upgraded to the paid version. The reason I did this is that you can add your own images and specify how they're split up--the default pictures are a bit difficult to solve, in my opinion.
KeyCaptcha was the only one that worked w/VM and it works great. It stopped "all" the junk registrations, I was getting about 10 per week.
During the last VM upgrade, I wasn't getting any Captcha on some pages. I contacted support and they went in and fixed it within a day. Nice product, nice support, nice results.
Thank you for your excellent review
The only reason I gave it an "average" review is that it doesn't support IE7 e.g. if you try to submit a testimonial in IE7 and hit the submit button but have forgotten to fill out any of the required fields the submit button becomes grayed out and inactive. You have to reload the page/form again and start from the beginning--you lose everything. This is also evident on their demo site.
I contacted support and they said they'd fix it in some future version. So, until then, just keep this in mind. Other than that, it works great.
The two test sites, 1.5 & 1.6, I tested it on had basic, custom templates and various plugins/extensions installed. Akeeba handled all of them without a hitch. I even took a backup and installed it on a different server/database. Again, no problem proving how easy it is to migrate to another server. I can now offer a very competitively priced backup option to all my clients for the Joomla sites I've created for them.
A breeze to install and run. Download it now, install it, back up your site, then donate to the author. It's way worth it and will make your Joomla development life so easy.