After reading your post the first thing you need to do is get hold of a copy of the internet policy and read it. If it says that viewing pornographic material is gross misconduct then your chances of getting off with it are very slim. you should also look for anything that states you should report any inappropriate emails, although why you would open your personal emails at work beats me, as any tracking software would log your details and password for it.
As SarEL says there is no excuse for allowing someone else to use your works laptop or even your password, the laptop is given to you to use with trust and your password is so only you can use your works account. Your company wont monitor your laptop, only your account.
Everything you do with a pc is logged inside it, aswell as this companys sometimes use tracking software to monitor your pc usage. This will log details of what programs you have used, where you have been on the internet and also how long you have spent on each page you visited. You have said that you work for a large company, so i will assume that they will use some sort of internet tracking. software
As MrBlack says mousing over an image will not download it, but the tracking software (if used) may log it as being viewed.
7500 websites in 2 months on a works pc does seem excessive to me, have you been link hopping? as this is the only way i can see how you could log so many. I would definitely ask for a list of the sites that you have been visiting. When you get this list look for items like; google.analytics, this isnt a site but a tool google uses and some company's may try to pass it off a a valid site, also look for any sites that have the same name, as this could just be different pages of the same site. You will also need to go through this list to see what sites you have visited that are something to do with the work you carry out.
The email you opened should have been reported as soon as you saw what it contained, that way they may have at least overlooked it in the forthcoming investigation.
Lastly, although it would be a weak argument, it may be worth asking if your company uses blocking software, if they dont it may be a very weak straw for you to cling to.