You can have Mif2Go use the original referenced graphics files, instead of the files generated by the FrameMaker export filters; and you can also eliminate spaces from the names of those original files, to make the names valid in all environments:
If you find that some of your referenced graphics are still being exported when UseOriginalGraphicNames=Yes, look for artifacts in their anchored frames. In FrameMaker, select an anchored frame, then on the right-click context menu choose Select all in frame to see what turns up. For example, if an anchored frame contains two .jpg images, includes callouts, or has a border drawn with FrameMaker drawing tools, Mif2Go generates a graphics file using FrameMaker export filters. Also look for empty Text Lines, and objects outside the boundary of the anchored frame; such objects are easy to spot in a MIF representation of the document file, and can be deleted in MIF.
The <img> tag references the original name of the graphics file (possibly modified for path and extension by several other settings acting in concert), and uses the FrameMaker-determined size of the original imported image as opposed to the size of the enclosing anchored frame (to preserve scale, also possibly modified by other size settings).
When you set UseOriginalGraphicNames=Yes, you are promising that there are graphics files (Mif2Go does not check!) to be loaded by the resulting .htm using whatever you set for the graphics path and extension; see §126.96.36.199 .
The size of each graphic will be the size of the actual image, instead of the size of the anchored frame in which it was displayed in FrameMaker. For example, if you put a 2" x 3" graphic in a 7" x 3" frame to center it so that there is no runaround; and you export the graphic using FrameMaker graphic export filters; the resulting exported graphic is 7" x 3", and mostly whitespace. When you use the original graphic, you do not want it stretched to 7" x 3", you want it to be 2" x 3".