I just started modding DOD not long ago and i'am trying to create my first mod. Since i just started modding,i will encounter problems and trying to get the thumpnail for my skills and items is one of the problems. So pls help!
Your sprites folder, and every other folder needs to be side by side with the "mod" folder, not inside it.
I haven't read through your mod yet, but if its still not working after you take Daynab and Monkooky's advice makes sure that the filenames and directory names are the right case. That was something that screwed me up before; I didn't know they were case sensitive, but they are.