The ones that are unreadable are likely the words from the texts. Those words do not count against you. You only have to get the real captcha correctly, which

The ones that are unreadable are likely the words from the texts. Those words do not count against you. You only have to get the real captcha correctly, which happens to be the clearest word. Try experimenting by entering just one of the words. I had no difficulty with these, while I usually have a LOT of difficulty searching vBulletin forums I am not a member of. I would write "If you're having difficulty, enter one of these words" at the top. The user will likely enter the clearest.