When you import items with pictures into the 4D Business Kit Items table, you can specify the picture size you want to create. This is to say that, for each subsequent import, the picture size during the import will only be applied to that session. This is regardless of previously imported Items (previously imported Items won't be affected, and will keep their original size properties).
If you have lots of different items with different picture size requirements, you may group them together and import them during different sessions. This gives you the option of controlling the picture dimensions of each import for a group of products.