"If you can't read that, your an idiot!"
Hmm...really? Your WHAT is an idiot? If you write in nonstandard English, you will always be misunderstood. Your simple sentence is in nonstandard English. It is nonsense.
"your" is a possessive form of the pronoun, "you".
"you're" is a contraction for "you are."
If you wish to call someone an idiot, please do try to use correct English, or you won't be understood.
Thank you for your attention.
Okay, so how about
"If you cannot read that, than you're an idiot?"