The easiest way that I've seen (and the way I do it) is to create a 2-bit *.bmp file.
Note, it's very important that the file is actually 2-bit. Many programs will tell you that the file is 2-bit and then go ahead and save it in a higher bit format. Irfanview is one program that will actually respect your format.
Once you have that file, you can open it in Windows Paint (again, this is fairly important), CTRL-A, CTRL-C
Then in altium designer, select the layer you want to paste the image onto, and paste. You can paste onto overlay, copper, or any other layer type. Any pixels that were black in the bitmap will come out as filled. White pixels will be empty.
Once your image has been pasted in, you can turn it into a union and resize it as you like.