Hey,
Sorry for not replying sooner.
I’ve been thinking this one through …
The simplest way would be to replace the file on your website, you will need access to the plugins directory either through your management tools (normally a cPanel) or FTP access. The big disadvantage here would be when the plugin is updated you will loose your image and have to upload it again.
You could also upload an image to another location, like your themes directly and add some CSS to your themes styles.css file to change the style of the infobox.
For example
li.gform_infobox_helpn {
background: #9fdaee url("../path-tofile/file.png") no-repeat scroll 5px 10px;
border: 1px solid #2bb0d7;
}
Note that #9fdaee is the background colour and #2bb0d7 is the border colour.
With both of these options the images would need to be pretty much the same dimensions (48 px by 48 px).
I’m not 100% happy with the images I’ve used. And would actually like to provide an interface to choose different styles. If you can find a set of icons that covers each of the types and is royalty free, copyright free etc. I would be more than happy to try coding it into an update.