Draw a guide; I'm not sure if the paint program you're using supports layers, but paint.net is free and includes that feature. You can draw a 32x32 block layer, (doesn't need to be pretty -- could just be a black filled box) and then delete a character-sized hole from your layer. Set up your paint program for a 32x32 grid. Move your layer to check and see that the character is centered in each 32x32 frame. If not, select the character layer, use the selection tool to grab the character from his 32x32 frame, and move him to fit in the character-sized hole mentioned earlier. Repeat for each frame.