IE is not the only browser that allows pixel units for font. Read the CSS Specs. I think IE may not interpret pixels as a relative size, as it should. But, it is perfectly valid to use pixels for font size. However, since IE doesn't interpret px as relative, it won't allow resizing. I generally recommend one use em or percentages for font-size.
If an element's dimensions are given in pixels, then resizing font won't affect the element's dimensions. If, however, you use em for the dimensions, resizing text will affect it. Percentages used for dimensions will size the element according the it's parent element. Thus, if you have the body element set to a width of 50%, and since the html element defaults to the width of the browser window, body will be 50% as wide as the window. Hope that made some sense.
In terms of images replacing text, I'm not generally fond of it. The practice can be considered acceptable as long as you're not replacing large amounts of text, and only if you're using it so that you can implement a non-standard font. For instance, say you've downloaded a really cool font. But it isn't standard on computers. You can make images, and using the proper techniques, replace headers with it. Headers, and site nav. But anything more than that is pushing it. And for cross-device compatibility and search engine rankings, I do suggest you use image-replacemnt techniques. If done properly, it will work fine.
