Pixsy
Pixsy (previously called Image-To-Bitsy) is a tool for building tile-sets and rooms from images.
When I first started using it, I found that often it would create images with background where I wanted foreground and vice-versa. This matters because in earlier versions of bitsy, if you want to hide your avatar in such an image, you need to be able to make the first and third colours in the palette the same. If pixsy has created your tiles the wrong way round, you end up with your avatar looking like a block of foreground colour, not background colour. This is less of a problem now with bitsy 8, because you have two new options, you can control the background colour of your avatar, or you can choose a different avatar per room.
Here are two images that look identical:
But when put through pixsy, the results are different:
Here using the standard blueprint palette for bitsy, the darker colour is the background and the lighter blue is the tile colour.
The first of these results is what I want, not the second.
The difference is in the source images. The first has a transparent background and the second has a white background. And it's this which is causing pixsy to return different results.