Just brainstorming:
Backend at the moment
With using userdirs you most likely want to store the whole path of the image in table. Would say in all accounts that eould be better. This way for example the userid can be in the path and makes things easier. But using ids will also give some insight to others about what usersids are on the site.
When user is logged in he/she will upload image. Onsave the id of user is recognized and attached to image-path. If imagefolder of user does not exist it will be created.
Images are for example stored in
images/jem/ud/165/events/
As talk is about 3 images i do wonder if it makes sense to use "small" as folder. Maybe thumbnail would be better. In events folder the normal one goes and within thumb we do output the thumbs. What can be done is to prepend the small ones with an eventid number and width/height numbers.
A problem can be when later on there is decided not to use userdirs..or when using imagedir from the beginning as there will be a userid check so previous images won't be there. With exporting/importing you've also to make sure that the user does exist.
Ah well just thinking...