What version of Inventor?
Image files must be associated with MS Paint. (search here)
>I really want to put .gif files on with transparent backgrounds
Doesn't have to be gif to achieve transparent. You can control the Properties of an imported image and the lower right pixel in image is used to control transparency.
-----------------------------------------------------------------------------------------
Autodesk Inventor 2019 Certified Professional
Autodesk AutoCAD 2013 Certified Professional
Certified SolidWorks Professional