Ahh. :3 All those things I've had /bad/ experience with.. haha.. Except SAI. x3 :)) Well, you know.. In making a new canvas, there should probably be something like choose the background color.. Which will be transparent so you won't have to worry about the transparency. Then apply the width (350), and the height (19).... Then you fill it with a desired color of your choice.. And such. :3 Umm.. I think this works in Pixlr... But, I dunno. Still, if you can't make it transparent, there's always Iaza. ;)
XoXo. :))