It will delete anything that snapchat has stored on your phone. If the photos are stored in the cloud then you should be ok. The way to find out is to sign-in to snapchat on another device and see if your photos show up there if they do then they are stored in the cloud.
Any photos that are stored on your phone will be erased. Also you will need to sign in to your account again.
Be careful, better safe that sorry. If you don't want to risk losing you photos then don't do it. You could look for other things to delete.
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