Generally speaking you will need to ensure that you have the correct level of permissions to accomplish the task at hand. I'm not sure if setting the folder to have 777 permissions is the best way to go about this but you will definitely have access to the folder if you do this.
Permissions are setup differently depending on the server and host, with many there is no need to add these extra permissions to get the images to upload. If you are unsure about the permissions you have applied and the type of security risk it could pose you should speak with your host to see if there is a better method of allowing read and write permissions to this folder based on their setup.
I think that most of the time when you are setting the permissions to these folders it is just the web user that has the permissions and so long as the permissions are only to a web accessible folder this should not pose too much of a risk.