1. Use a image editing program. Paint works but everything is very low quality, so I'd recommened Phototshop. Or, you could ask someone to make one for you.
2. Open the file with aforementioned image editing program, go to file, select Save As and change the format. Again Paint works but is not recommened due to it's poor quality.
3. Yes, it'd take some time but if you have the skills you could theoretically modifiy them into green.
