nice program, a few suggestions for possible improvement:

1)   Left-click on tray icon should change the current wallpaper to the next in the list - several other wallpaper apps have this function.
     
      The current way to do this is have the GUI on display, which gets in the way of seeing the wallpaper, or continual right-click +
      select on the tray icon menu, which is a bit of a time waster

2)   where you list "path" to the folder in the upper left there should be a way to click that folder and then below it each file is listed
      and then when we click on a file we see in another part of the GUI a small thumbnail of the file, maybe 1x1 inch or 2x2 inch,
      enough to recognize the file, and maybe right-click delete it also, or copy, etc - the usual file right click options

3)   GUI ideally should be re-sizeable


i'm not a programmer so i dont know how difficult each of these would be to implement, would be interested in hearing your reply.
BTW i'll donate $25 paypal in advance to your Skwire website if you can make items #1 and #2 happen