Try flashing it again but perhaps update the picture.bin time stamp to a more recent date. I don't know if your dates are updating when you copy the image to the SD card or even if it uses the date of the file vs some header info.
That's worth a try. I'll let you know how that works out. Since it seems you can up- and down-grade just by putting files on the SD card, I don't think that should be a blocker, but I'm game to give it a try.
No luck. Still the same behavior.
I noted that the update 1.7.2 came with a file named filename.txt, that listed (in Chinese, thanks, Google Translate!):
Target file: D:\HP3500\HP3500_picture\binHEBING\Raw\picture.bin Size: 0x00017510 The parts are sized as follows:
Index File Name Size Offset Force Insert
1 D:\HP3500\HP3500_picture\binHEBING\Raw\1LR.raw 0x00000780 0x00000000 No
2 D:\HP3500\HP3500_picture\binHEBING\Raw\2R.raw 0x00000780 0x00000780 No
3 D:\HP3500\HP3500_picture\binHEBING\Raw\3L.raw 0x00000780 0x00000F00 No
So, file offset starts with zero as if there was no header, or the code accounts for the header, and the last entry:
55 D:\HP3500\HP3500_picture\binHEBING\Raw\60dot.raw 0x00000080 0x00017490 NoStarts at 0x017490 with a size of 0x80, which adds up to the size of 0x017510 referenced in the header. That's 95489 decimal. The file is 95,505 bytes, so there's 16 bytes unaccounted for, if I did my hex math correctly. It might just be padding, but it could be a checksum or version tag. A hex editor doesn't show anything I'be been able to work out.
My latest emails from ecowitt support -- they get a bravo for quick response! -- suggested trying another SD card or reformatting the existing one. Most of my cards are new Class 10 and I thought perhaps too fast for the SD reader firmware (this might be what started the problem). So, I reformatted the same 2Gb card and had the same results. Afterwards, I checked the filesystem on Linux and it complained about bad checksums on the long file names for Firmware.bin and picture.bin. That may have nothing to do with the problem, however, as lots of software mis-use FAT32 LFNs, especially if they stick with 8.3 filenames.
They also suggested trying a card with just picture.bin installed. I did that, both on the 1.6.9 firmware Olicat had pointed me towards, and also the latest 1.7.2.-1229 firmware but, alas, no change in the symptoms. There is no display of "Updating Picture" but it does show "Update Firmware" and then restarts, blinks a white screen twice, and re-displays, but without graphics. The sensors are getting picked up, the datetime is resetting properly, but there are no bitmaps for SD card, battery levels, etc. The "About" screen does confirm the firmware "took."
Finally, after nine tries with a combination of all of the above, I got yet another new 32 Gb Class 10 SD Card, re-partitioned it, reformatted it FAT32 with full erase, copied Firmware.bin and picture.bin to it. I updated the datetime on the picture.bin to today's date and time, plugged it in the HP3501, powered-down and -up and... "Updating picture..." appeared and "Updating Firmware" and the machine restarted and is working exactly right. Which did it? I'm inclined to think the new card, as I had tried fiddling with the file date before.
Thanks to all for your interest and suggestions!