Hm. The way it works, it should always get images from the media library, that is, when pressing the button next to the text field, it ought to open the media library dialogue box. Then you just navigate to whatever (previously uploaded) file you wish to use, and select it. The text box with the image should get filled automatically with the correct path, you shouldn’t be putting anything inside it manually (because WP installations are different, e.g. single-site or multisite will have different directory layouts; that’s why I use the media library dialogue, which will figure out the path correctly for you).
There is an extra step: once you’ve selected the image, you need to click on “accept” first. Only then will the image file be copied to the proper place for you to add your imagemaps.
From your description it looks like you’re trying to add an image file manually. That’s a bad idea. Use the media library! It’s waaaay simpler than having to figure out image names and paths…
As for the file format, this plugin will be happy with whatever image format the media library uses. So if in the future WordPress (or web browsers in general) supports different file formats, they will always work. Just upload your image to the media library: whatever “magic” WordPress does to your file to make sure it remains compatible with WP and the browser, will be enough for the Imagemap Selector plugin. That’s why I prefer to use the media library to upload files!
