I tried
openwrt-18.06.1-brcm2708-bcm2710-rpi-3-ext4-factory.img and the sysupgrade version following instructions to create the SD card.
I have a display plugged into the HDMI and when I power up the color swirl appears with a yellow lightning bold in upper right corner.
Almost immediately the red power LED goes out and the green LED next to it repeats 4 slow and 4 fast blinks.
I'm thinking I need to replace the contents of the boot partition with the contents of an SD card that boots. I'll try that.