Whatever you like - family, house, views or interior. We would suggest that the main photo is either the outside of a house or inside of an apartment or a view iff that is best suited. If you are uploading these yourself they will need to be in either jpeg or gif format. A stranger is often a friend that you just haven't met yet. There is seldom a problem between exchangers.
Most exchangers look after their exchange homes better than their own.
And as most members are professional people with high standards there is little to worry about. Remember that they are letting you into their own home. Get your friends and neighbours to meet them and look in on them and and make them feel part of the local neighbourhood. Another benefit is that by having your house occupied there is less chance of break ins easing worries many of us have when we leave our house unattended. You will need to have access initially to enter your property details.