I have now uninstalled 10.4.2229 and reinstalled avast 10.3.2225.
Points of note: I couldn't do a clean reinstall as the avastclear.exe threw up an error, not compatible for this system (or words to that effect) or the file is corrupt (yet it passes a CRC check). I guess that this was a new version to cater for 10.4.2227 and later beta builds.
So I decided just to install 10.4.2227 and then update to 2228 - but I got the same error as I did with avastclear.exe. Ended up installing 10.3.2225 and started getting issues with images in yahoo.com again.
I Disabled HTTPS scanning, stopped and started the web shield, opened yahoo and whilst the images now appeared, but despite https was disabled avast was scanning them.
There are now a large amount of topics/posts relating to this non display of images and =this issues isn't limited to XP as was first thought - see
https://forum.avast.com/index.php?topic=176073.0 as just one example.