1franka, you have to delete from the_skin ( by pongo ), \ux_nextgen\res\the_dark\Xdpi files : loading.bmp , loading_progressbar.bmp and loading_progressbar_end.bmp
Copy into ux folder, the file below.
Also add in sys.txt :
Spoiler: ux-utility loading
Then, if everythings are OK, using Total Commander, replace .bmp with yours :
320_240 => ldpi
480_272 => mdpi
800_480 => hdpi