After you have successfully converted your document, to experiment with different graphics DPI or format settings without actually producing document files, check Write only graphics, no text in the Mif2Go Export dialog. When you check this option, Mif2Go uses the .grx files from the previous conversion run to get a list of graphics to produce. You must have already run the conversion at least once to produce the .grx files, because this option works only when .grx files are present in the project folder. See §C.2.2 .
If you really want to output only graphics, without first producing .grx files, you could set [FDK]GraphicsFirst=Yes in the configuration file; then instead of referring to .grx files, Mif2Go would crank out every graphic in your document, including unused reference-page and master-page items (but not any graphics in hidden conditional text). See §126.96.36.199 .
This process is really useful only for RTF output; for HTML and XML, setting [FDK]GraphicsFirst=Yes is usually a total waste of time and space. And if you decide to change graphics formats, you must run the entire conversion again (possibly with Do not write graphics files checked) to get the <img> src attributes right.