Irregularly shape background .bmp

[DestoFante]

Hello!

I am very new to CC3... but very determined to make it work for me! ;-)

I bought the software a few months ago, I played around, and had a great time - I actually succeded in making a few maps I was very proud of! Then my computer crashed (for unrelated reasons), I got frustrated, and I left CC3 alone for awhile after I reloaded it on the new machine.
Now I am back... and I cannot re-do a nifty trick I achieved on my first go! Here's my issue.

1. I start with a map in .bmp;
2. in MS Paint, I cut my map with "select- free form", so to give it an irregular shap at the contours;
3. I saved the new .bmp file (attachment 1);
4. And import it to CC3, as a nice background for the map I want to create.

And here's the problem: when I first play with it 4 months ago, I was able to import the irregularly-shaped imagine into CC3, as irregularly shaped! Now, see imagine 2, it comes as a squared imagine with a ugly white at the corners. I cannot recall how I did it the first time!

Anyone has any help to refresh my memory? Thanks is advance for your help!

DestoFante