1.) You don't need the converter.
2.) After you do a printscreen, you need to paste that into a program. I personally use Gimp - It's free and it's got a lot of great features. So, what I would do is open a new document in Gimp. Turn on the grid and make sure it is set to 32 x 32 (Show Grid is under View, Configure Grid is under Image).
Once your grid is on, paste your print screen. Line up the corners of the background of the sprite (blue/green for me) with the corners of the grid. Use the crop too to crop around all but the left column, using the grid as your guide. You can also turn on "Snap to Grid", under View, to aid this process. In Gimp, you will then want to export the file (not save).
Also, you do NOT have to erase the background. When importing the file into RMA, you can select that color for the transparency color.