The more images you have, the more space is used slowing down devices. Images can be deleted from the device or the web version.
Deleting images one at a time
From the device or web version the image can be erased one at a time when editing each product: Products > select a product > tap the image > Delete Photo. The image will be deleted immediately and removed from the server after syncing.
Deleting unused images
When products are deleted or images are uploaded, there could be several images without products or not linked with any products. Such images can be deleted from web as follows: Settings > Restore > Product Images