if you go to the preferences side of the widget, select the display tab at the top of it and then click on Hard Drives. Make sure your Mac's HDD is selected in that window.
If you look at the attached image of the OP you will see that's not what he is asking.
good point; been a long day.
Does the OP have a custom icon set for the main HDD?
this is odd - the OP has custom icons for all his disks yet one (the main HD) does not display properly.
My HD has a custom icon and displays the custom icon in iStatPro.....
ooops.....
Anyway - it appears to be the # sign that is causing the problem - I stuck a hash sign in the middle of the name of my HD and voila - my icon looks like yours - ?.
So remove the # from the name and the icon will show correctly.
got rid of the hash...now the image shows up. now i look stupid.
Why ?
Seems odd that it displays the name (with the #) and the disk details but does not show the icon.
It's sorted now and it did not cost anything !!