It's been a few months since Microsoft released new ISOs for the Windows Insider Program, but there's a new one. You can now download a fresh image of Windows 10 Insider Program build 19035.
Microsoft also added a new channel to the ISO download page. Previously, you could only get it from Fast ring and Release Preview ring channels, but now the Slow ring has been added. At this time, the Fast and the Slow rings are on the same build, but this is set to change. Microsoft has made it clear that it's prepping the Fast ring to start receiving previews of the next Windows 10 feature update.
ISO images are useful if you want to clean install a Windows 10 Insider Preview, and test out the out-of-box experience. Typically, Microsoft releases a new one every month or two, coinciding with the latest builds in whatever ring they're for.
If you want to download the new ISO images from Microsoft, you can find them on the Insider download page here.