Thanks. The program is designed to convert thousands of tiff files to png after performing several operations on them, and the compression needs to be lossless, so no jpegs.

I did finally find a set of default arg values in the Imager source code, and one of them is compress => 1, but changing it makes no difference in file size. I saved a few images with GIMP using different settings, and judging by file size Imager uses png compression level 6. Also, from the same tests it looks like going to maximum compression from 6 will not give more than 4% improvement. Since I'm already getting better than 3:1 reduction in file size by converting to png's I'll just be happy with that.