I have an OPPO A3s where there is an option to clear image cache approx. 900 mb. size.
I want to know if clearing it will delete my pictures or not?
The device should only clear the thumbnail cache which is used to show the images faster in the gallery when you scroll. It is also used in other places such as file manager.
The cache will be rebuild again unless you reduce the number of images on your device. So, deleting it adds very less practical benefit.
Also, it is a good idea to take a backup of your photos before touching them. Do not risk your data just in case the app has some bug.