These assets are still stored on the site and accessible via direct link (eg. a previous group icon of mine). Thumbnails/icons for experiences can be deleted from Place settings (https://www.roblox.com/places/[place ID]/update#), but group icons still exist and it seems the only way to currently find them is through a direct link.
Every image on Roblox (at least ones that can be seen/used in games) has an image asset behind it. Everything you see on Roblox is just a user facing asset used to handle everything, while ingame they all just point to image assets.
For example if you paste in a decal asset ID into a decal, it will change it to the relevant image asset ID.
A decal ID would be 2023871 while the related image ID would be 2023870. Same goes for shirts, a shirt ID would be 1826151 while the image ID it uses in game would be 1826150 (this is also how clothing is copied, users just download the image asset)
To answer OPs question, there used to be a tab for images under development items, however it’s since been silently removed at some point for unknown reasons. I know it existed at some point because I do remember seeing and using it, but I can’t find it anymore no matter where I look.
It should 100% be brought back though.