The PNG is probably fine, you are probably doing the command line process wrong. You have to navigate to the proper directory in the CMD window, which I hated. I use batch files to do it now. I wish I had known how to do them sooner. I cant tell you how many command line conversions I did at first.
Make sure you have the png2bmp in the folder with the png files. Then Right Click > New > Text Document > save as deck.bat. Right Click > Edit the BAT file. Paste in this code:
Code:
png2bmp deck.png deck.bmp deck.a.bmp
png2bmp cardrank.png cardrank.bmp cardrank.a.bmp
Save the batch file, and run it. It will automate the command line process, and output the 2 bmp and their masks. You can then delete the PNGs and BAT/EXE files if you want.
Here is a screen of some ctrls I made yesterday using the same process.