After a lot of trying I finally got the display to download (upload!) a start screen image. However, it is not showing. The picture is good, at least, it shows well with drawImage(). hat are the constraints on the image when used as a start screen image? Which colour mode? Any size restrictions?

See the sample using our online emulator:

https://www.digole.com/tools/digole-emu.php

Click on: "Splash & commands set", you can load your image into emulator, and produce the complete data from emulator either

I am afraid I don't see what you are pointing at. I see the drawImage() ("EDIM1") being called, but  that, as described above, I have done succesfully too.

The trouble I am having is with Download start screen to module(SSS).

I Checked your order, your module doesn't have flash chip installed, so, there are only 16KB space for welcome screen, means if you tried to write over 16KB data to welcome screen memory, then the module will reject for writing.

Also, the "EDIMx" functions are slightly difference in Welcome screen (Commands set) mode and regular serial mode, you need to remove the (x,y) out of these commands, and use "GP" command to predefine that.

"I Checked your order, your module doesn't have flash chip installed, so, there are only 16KB space for welcome screen"

That's interesting, I hadn't figured that out myself. Anyway, the image counts 100x125 pixels, so 12500 bytes. That ought to fit then.

The EDIMs requir w*h to calculate the data size; likewise wants SSS the data size explicitly supplied. Mistakes with that freeze the display for it keeps waiting forever on more data.

By all that I can judge, the data transmission runs well now. The program continues and the screen remains responsive. Only the screen stays black after start-up. I did issue a DSS/1 for the sake of it.

Yes, use "DSS1" to enable to display welcome screen when power applied.

Would that be like you write "DSS1"  or "DSS\x01" ?

Anyway, neither works for me. And the example welcome image is 30kb, so no point even trying that.

Could an image be stored on the mini-SD? Would that be loading faster than through the programmers interface? How is the mini-SD addressed?

The SD socket on the display is disconnect to on board MCU, you need to drive it yourself, Digole only fan out the pins on the SD to DIP header (6 pins)