Fast Dial version (3.6) and the previous version(3.51) have a known limitation (because of a bug in Firefox itself) of transparent logos not displaying properly if they are not of a 4:3(ex. 400x300") ratio.

If you are upgrading from a previous version and have only logos of the 4:3 ratio, there will be no problem with them displaying.
However, if you have some logos not of that ratio, they will display as stretched.

To remedy this problem, either download a logo from userlogos (they are automatically in the 4:3 ratio), or possibly make one out of an icon/inage of your own.
Tutorial here.