I know what you can do with MSPaint... It kinda is what produces everything I do.
Anyways, Transparency as in making it so a specified color will not show, but instead be the background color? Like my sig, the border ones? Ya, I did the transparency with MSPaint. (Except the very top one, that was done by KiLLeR2001) Sure I used a different program to make them GIFs (My MSPaint when I make them GIFs mess up the picture, so I use a different program just for BMPs->GIFs/JPGs)
EDIT:
See on my sig before the "AutoStacker" thing it has an image that has a black line on the left? I didn't make that one transparent, so you can see that the rest are indeed transparent. I did transparency in MSPaint
