While I agree with Jeff and Josh that open rates are not 100% reliable, i think you can still gain some insight from a quick analysis--especially comparing your campaigns against themselves independent of your A/B testing. After all, people generally don't change their image control status on a day-to-day or message-to-message basis. The same ones that blocked your images yesterday likely blocked them today, and will probably block them tomorrow. If your open rates change using the same list, your text, subject line and/or image tags are having a strong influence on that change--its probably worth poking around a bit to see what the differece might be.
--If you are seeing a slow but steady improvement in open rates (and aren't varying your list or image content), then your trust-level is probably growing as people click "Always Display Images from this Sender".
--You can check some of your higher-performing messages--see if they have subject lines and/or image tags that are enticing readers to click that "display images" link.
--Similary, don't panic if you have a low open rate for a particular message--look to see if its mostly text. If there are no images to see, it won't be reported as having been opened--people just read the text in their preview pane.
--You might also test readership engagement by sending a mostly-image message--something were someone would have to click "display images" if they wanted to actually see what you had to say.
The one thing i wouldn't do is compare your open rate to someone mailing to a different list.
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