If Ancestry.com is your main tree, then you should make your changes there. Don't try to make some changes in FTM and some in Ancestry. The sync process is still a bit fragile and many people have experienced confusion or failures or errors if you try to push it too hard. Don't let it do automatic syncing. Make sure you set it to manual sync so you can control when to do it.
If you want to make changes in FTM for something only it can do, sync your file with Ancestry.com, then make your changes in FTM, then re-sync before making more changes in Ancestry.com. That will give the cleanest results.
When you start a sync, you have the option to stop and get a report of the changes before it goes ahead and syncs (you have to be watching to catch it or it will go ahead anyway within a minute).
I highly recommend reviewing that file carefully and saving that report to a file or PDF. That way, you have a tracking of what was done and it can help if errors occur.
If you have or can make a small test tree, you might use that one to experiment with first before putting your "good" tree through the paces. That will give you time to get more used to the program and help avoid problems.
To answer your first question: yes it will download your photos from Ancestry.com if you choose that option. It will also download other images, such as census records, which is a nice feature :)