Today, Microsoft releases ISOs for Windows 10 Insider Preview build 18343. The images replace 18290, which was released back in December.
The new ISO images follow last weeks announcement to bring build 18342.8 to the Slow ring, marking the first time that Slow ring Insiders could test the next feature update to Windows 10, 19H1. According to the Windows Insider team, the bits in build 18343 are exactly the same as build 18342.8, so you should have no trouble staying on the Slow ring if you do a clean install.
In fact, the dropdown menu on the download page has the word "SLOW" in parenthesis, just so there's no confusion. As usual, there are a total of three ISO downloads. These include Windows 10 Insider Preview, Windows 10 Insider Preview Enterprise, and Windows 10 Insider Preview Home China.
To download the new ISO images, you can find them here.