Some photos are sharp on the phone but low-res on icloud.com/photos
We have discovered a workround for this issue:

Duplicate the problem image on the phone.
Use &quot;markup&quot; in the photos app to make some miniscule invisible change.  e.g a one-pixel black dot with a semi-transparent pen in some shadowed (but not completely black) region.
Save the modified image.
Wait a couple of minutes and the duplicated image appears on www.icloud.com/photos and is the full resolution version, as expected.

It's worth mentioning that just duplicating the image alone doesn't work: the duplicate will appear on the icloud web-page, but still be fuzzy low-resolution.  Also, modifying the duplicate using other editing controls like exposure, warmth, tint and so on isn't enough of a change to force the upload of a full resolution version, presumably because those parameters are held separately to the original image data (so repeated tweaking doesn't degrade the image).  My guess is modifying the actual image pixel data changes some hash value computed for the image and convinces iCloud there's something needing uploading.
It's a good enough solution for the handful of images we want to see properly uploaded at full resolution.
The most slightly annoying thing about it is that, while the duplicated and modified photos still seem to have the original date attached, www.icloud.com/photos now displays them at the end of the album and unlike the phone app there's no sort options provided.  Not a big deal though, as her main reason for using the iCloud website is just to download images to the desktop PC.