Could do it in Inventor 2012, but it is different in 2014 and I am having hard time figuring it out.
There are several see through materials provided with Inventor (for example, Expanded Metal 01, 02, 03, 04). If you want to create your own, watch the short video attached. The rules are simple - the image file name you are using ends with an ampersand (&) and the lower right pixel of the image used is the same color as the transparency.
AttachmentsCreateAndAssignTransparency.mp4, 2.83 MB