I was looking into it last night and yeah the PS3 store itself is still up, the images aren't as consistent as what's on the PS4 store though.
Like the main icon URL for Warhawk looks like this:
I tried using that link and swapped it with the Warhawk IDs and got this:
But the Sonic Adventure image looks great so I think I'll just setup a crawler and grab what I can. Hopefully most of the images are more like that. Warhawk was an early PS3 game so maybe that's why higher res assets aren't available.
From what I remember the thumbnails for the PS3 store varied quite a bit. Still it should be a good starting point for the PS3 games. I'm sure the rest can be filled in manually and the bad images can be swapped out with better ones just like the current icons.
So how are things looking so far?
Xbox One onwards - square achievement icons
Xbox 360 - square store thumbnails (tall thumbnails when square aren't available?)
PS5 - square achievement icons
PS4/PS3/Vita - square store thumbnails
Steam - logos on hero images (capsule images when those aren't available?)
GOG - ??? (the current icons cropped to square look like they might be fine?)
Origin - ??? (looks like these are cropped from tall images, so they should improve?)
Blizzard - ??? (3 games, I
think we can handle that)
Google Play - ??? (looks like these are currently wide icons squished to fit)
Ubisoft - ??? (looks like these are cropped from tall images, so they should improve?)
Stadia - ??? (looks like these are currently pretty good wide icons)
Epic - ??? (currently big wide background images scaled down)
Retro - square box art crop instead of wide crop, should mostly improve
Untracked/Manually Added - square box art crop instead of wide crop, should mostly improve
From what I can tell several platforms will be improved by the square crop without changing the icon sources, while a few like Google Play and Stadia may end up worse off unless there are more suitable images to pull for them. Even if there aren't it seems like less/better cropping overall.
I assume when square icons are implemented there will be a separate upload button for those on the back end?