I thought I answered this elsewhere, but maybe it was someone else asking?
GRS80 is an ellipsoid/spheroid, not a true geodetic datum / geographic coordinate system. If that's all that is listed (particularly for GRIDs or coverages), the software will create a "GRS80-based" geographic coordinate system. Because it's not a true GCS, we have no transformations to or from GRS80. It's more likely that the data is on NAD83 or one of its realizations.
If the raster's cell size is larger than a meter or so, you could redefine them directly as WGS84 UTM zone X and then project them to Web Mercator.
Or redefine the GCS portion as NAD 1983 (name may be "GCS_North_American_1983").
Or if you know that they're really on a different realization of NAD83, like NAD 1983 (2011), redefine the GCS as that.
Or use the create custom geographic transformation tool. You'll need the exact GCS definition from the raster. If there's no need to transform (you can treat NAD83 and WGS84 as equivalent (cell size > 1-2 meters)), use the null transformation method or the geocentric translation (leave the parameter values as zeroes).
If the cell size is smaller, try to do some research on what NAD83 realization was used to georeference these images. WIth that information, I can suggest what transformation to use.
Melita