Here's the picture I'm using:
https://upload.wikimedia.org/wikipedia/commons/thumb/5/5e/US-WhiteHouse-Logo.svg/1024px-US-WhiteHouse-Logo.svg.pngThere's blue in it, but I'm using pure blue all around the oval.
I saved it as a .bmp 24 bits and named it starting with a "{".
This generally automatically set pure blue as the last palette color index. (I verified and it did it.)
I did this process with other pictures and it works fine, but not with this one. (The pure blue is not replaced.)
I am actually using gimp.
I'm at work right now. I just have wally on hand. I used your Threshold Alpha trick and that was really helpful. I'll try re-adding the picture to Wally this way.
I'll read the tutorial later when I get back from work and try what it suggests also.