Max Num Chars Allowed In Function Param?

I have a page working perfectly that passes up to 10-12k worth of data into a function via an 'onClick' event. That onClick is one of the <img..>'s attributes, directly. In case later that (dynamic) data gets larger, am I toying with potential problams because of some limit - either a limit in what can be passed into a JS function, or else a limit in the max num of characters allowed in an element's attribute(s)? What do you think is the reasonable amount that I can pass before things start to break? I may need to go up to 50k. 100k maybe, but I doubt that much.