How to create a condition to reduce word count if photo attached

We're running a contest where we want people to share a memory in 140 characters or less. I have the field limit set up at 140 characters, but if they attach a photo (like in Twitter), the photo counts as 22 of those characters. How can we set this up in a way that makes sense?

See the bottom of the form I created (url below).

If I understand you correctly, if a users uploads an image, you would like the character limit to be reduced to 118. Directly this is not possible, but you can accomplish this having two different text area in your form. If the users uploads an image, you can hide the text area with the limit of 140 characters and display the one with 118 characters. 

If there is no image uploaded, you can display the text area with 140 characters and hide the other.

Please let us know if this helps.