In regards to browser testing, a screen shot isn't enough to ensure complete compatibility. You need to make sure things interact accurately (ex CSS Drop-downs, or javascript). To do a 'complete' test, you should install all major browsers on your machine.
I did that for most of last year. But you can only do it for so long since all you do is work, then go home and work. Plus it just takes so much longer.
If it's an idea you truly believe in, it may be worth making that your full-time job. But it is a risk, you just have to decide if it's worth taking.
Keep your software all up to date. For an example, I've seen an old version of a open source calendar be used to send 30,000 spam emails from a server.
Software like Fanstasico can keep your software automatically updated across user accounts.