[RazorFR]

That's maybe an issue with the skin. You will need to do some testing. Use the ROM context menu "Edit ROM" -- "Manage ROM..." -- "Choose default ROM artwork...". You should be able to link all the assets (titles, snaps, etc.) to any kind of official Kodi supported artwork (icon, fanart, poster, banner). Some skins display the poster on top of the thumb, even if the poster is the empty string ''. If so, make sure both poster and thumb point to the same asset (title I presume) that contains your imported thumbs.

I did some testing

At the rom level
Edit rom > Edit Assets/artworks > List
Edit Title (here I can see my poster not displayed in the skin)
Edit Snap
Edi Fanart
Etc...

If I choose anything else than title, get a message "directory to store xxx not configured". also there is no Edit Thumbnail, may be part of my problem.

I did the same at the launcher level
Edit launcher > Edit Assets/artworks > List
Edit Thumbnails (I see my poster, displayed in the skin)
Etc..

I also tried at the launcher level
Edit launcher > Choose default Asset/artworks
Thumbs (currently Thumbs)
Fanart (currently Fanart)
Banner (currently Banner)
Poster (currently Flyer) > Tried to change this one to poster but still nothing displayed at rom level

Strange thing is in a previous version of the AEL, roms thumbs were displayed fine after the import.

Yes! It's not a secret by now there is something big AEL-related going on. Actually, TTBOMK Hyperspin only supports 4 kinds of assets plus trailers. AEL is now one of the most complete front-ends out there. AL and RCB had many limitations/bugs that limited their use to small collections. Those limitations have been mostly dropped by AEL. Hopefully, soon there will be some skins that fully support all AEL capabilities and I believe many people will think of Kodi as a serious front-end for general use, on par with Hyperspin, EmulationStation or GameEx.

Kodi + AL is already a perfectly serious frontend (for those who just want minimum metadata + poster and fanart). AEL and also the Kodi retroarch version will probably convince + people.

You already have the virtual launchers "Browse by Title", "Browse by Genre", "Browse by Studio", "Browse by Year". ROM Collections is similar to Movie Sets, with the difference that you have to build up your collections/sets. If you want me to add a "Recently played" virtual launcher and maybe something like "Most played ROMs" with a counter of how many times you have played each ROMs that something I can do easily.

Yes recently played and most played would be a nice touch indeed. recently added not really useful I do agree.

Keep in mind there are two kinds of AEL users: A) The ones that have small, carefully curated collections. Usually user A) also has all kind of artwork (maybe even downloaded by hand on a ROM by ROM basis) and B) Users that have full No-Intro collections (thousands of ROMs per launcher), that download whole collections of artwork, and maybe have 2 or 3 different launchers (emulators) pointing to the same ROM directory (for example, I have retroarch picodrive, retroarch genesis plus gx and MAME pointing to my genesis No-Intro ROMs). I have to keep happy to both kinds of users!

Right, I guess I am more type A

The image is just my current game Menu/Hub in artic zephyr.