Icons for external HD

I would like to replace the genetic icons on my external hard drives. I have gone to the 'Get Info' and clicked on the icon in the top left and tried to replace the icon. Some times this works like a charm and other times nothing happens at all. I cannot seem to figure out why.
Any suggestions?
Thanks,
Ron

Kyle-K said:

If I had to throw a guess out there may be the icons are in the wrong type of file format.

Click to expand...

When I check some of these in Preview, they are said to be TIFF files even though they appear to be jpeg based on their names. Could it be that?