if you want to edit an image and change its font, yes you have to host the fonts on your server... that't the way the fonts are managed with the Registration Captchas... (if you check vBulletin well)

if you want to change the style of the text, not the images, you have to set the proper font names in CSS, not on your server... if set well, users will see the text in proper fonts or the replacements if the fonts are not present in the client's computer.