If you are only interested in converting a raster to a format that can be downloaded onto Collector then try creating a tile package (TPK) and side-loading that onto the device. Rasters are not support in vector tile packages.
For more detail, do see Go offline—Collector for ArcGIS (Classic) | ArcGIS and https://community.esri.com/thread/90177
If I am not editing data, then I usually create a TPK of the raster data, then include that in a MMPK with my vector data (with pop-ups and bookmarks) that is easily side-loaded onto Explorer for ArcGIS.