In a previous blog post I discussed using Picture Controls to host lots of images within InfoPath and manage the tooltip.

InfoPath FormServer renders picture buttons with a pop-up Picture Icon that will hide any alt-text that you've set on that picture.

The key is in the Core.js file, which you should NOT modify.  As this is what could happen if you modify core javascript files.

So instead, you should create a separate webpart page, put the Form control on that page, and add this script override for the LinkedPicture_OnMouseOver function. 

The key is "if (a.disabled) return"

Then in your InfoPath form template, set the control to read-only will ensure the Picture icon doesn't appear on hover, allowing the title text on your image to show up on the browser.