Regarding email clients... Marketo's out-of-the-box email reporting gives you a high-level overview of your email's performance, but doesn't really dig any deeper. If you use an additional service (I use Litmus, but there's others out there, most with free trials), you can gain considerable insight into your audience. For example, I sent out a webinar invite that had ~4,500 opens and with Litmus I'm able to find out that of those opens, about 30% were using Outlook (and it will break it down further to which version of Outlook), 25% Gmail, 18% iPhone, etc. This information is very helpful when designing, because each client has its own quirks for how it renders HTML, so I recommend prioritizing them, while still making sure your emails will render OK in all of them.
For images, most email clients by default do not load images immediately, unless they're in your address book. So this means that if you lay out a beautiful image-filled email and I open it in Outlook, I won't see anything until I click the warning saying images weren't loaded, and choose to load them. This also affects your open rate, which is why that number is pretty misleading and you should focus more on clicks instead (opens are only recorded when images are loaded, or a click is registered).
I also should have mentioned that when it comes to deliverability, it's very important to avoid spammy practices. There's a lot of debate on what constitutes spam, but a tool like Litmus or Email on Acid can help with that too (it will run your email through spam filters and let you know if it thinks it will fail or not). Generally you need a good balance of text-to-image, avoid things like $ dollar signs, ! exclamation points, % percent symbols, the word FREE, using lots of caps, etc. You can get away with a few, but I prefer to reduce my risk.