integer'image(some_int) results in "Expression is not constant" error

I'm actually not using a ROM or RAM for any of it. There is a giant std_logic_vector that dictates which pixels should be on or off for any given character. Then, to be able view non-character variables like integers, I have that ridiculous, giant case block which is used to convert an int to a string. I had a more elegant solution that actually performed some math to figure out, digit by digit, what the string needed to be for any given int but it didn't like that for reasons I can't remember and I had to use the giant case block instead.