get relative URL instead of hard-coding

The plugin I’m working on works fine on our site, but I plan on releasing it for others to use as well. How do I get the relative URL to pull an image instead of hard-coding the image as it currently is below:

echo "<input name=\"submit\" type=\"image\" src=\"/wp-content/themes/WMG/images/compress.jpg\" class=\"css_compressor_btn\" />";