Main thing I get from googling that DLL is that allegedly gimp 2.8 needs a 64 bit system, won't run on 32 bit. I don't use gimp, but I have Paint Shop Pro 7 and could never get the DDS plugin to work on that.
http://www.mwgfx.co.uk/
I use DXTBmp (sixth one bottom row) to open DDS files, since that opens practically any image file and splits it so you can send either the main image or the alpha channel and send them to any image editor as a BMP temp file. After editing either or both save as DDS and you're good to go without any plugin. Works with any image editor, even plain vanilla Windows paint.