I'll get in b4 anyone else suggects the DS2 Skin thread - the OP has already been there earlier....
Anyway, there IS no 'simple skin maker' as such for the DSTwo - only Skin makers that show you what your chosen graphics will look like on the DSTwo, but what I can suggest is .....
You copy one of the existing skins (such as a 'plugin' one or 'UI') & just replace the existing graphics with identical sized graphics of your own choosing - then copy back the graphics onto the card (but remember to make a backup of the original graphics first)
IF the graphics look 'corrupted' on the DS then those particualr graphics usually need to be save as 16-bit (the corruption is because they are actually 24-bit) - & GIMP is a program that can do that
As for the iMenu graphics - the bare minimum you'll need to do is change are the graphics 'top_bk.bmp' and 'lower_bk.bmp' inside the imenu folder to change the appearance (clock graphics may need a little more work though)