Changing image formats for your HTML5 and CSS3 web pages with IrfanView is really easy. For example, find an image file on your computer and follow these steps to change image format:
Load the image into IrfanView by dragging the image into IrfanView or using the File→Open menu command.
Make any changes you may want to the image before saving.
Use the File→Save As command to save the file.
Pick the image format from the Save Picture As dialog box.
Save the file with a new filename.
Keep the original file and save any changes in a new file. That way, you don’t overwrite the original file. This is especially important if you’re converting to JPG because each successive save of a JPG causes some image loss.
Don't use spaces in your filenames. Your files may move to other computers on the Internet, and some computers have trouble with spaces. It's best to avoid spaces and punctuation (except the underscore character) on files that will be used on the Internet. Also, be very careful about capitalization. It's likely that your image will end up on a Linux server someday, and the capitalization makes a big difference there.