image read in vhdl

For a start, this is never going to work. It looks like you're trying to write sofware. This is clearly not the full code as there are lots of things missing (like a process, architecture, etity etc).

So errors in your code: row_pixel is an array of length 1. Your for loop also never executes as you have 0 downto 3 (a null range).

I suggest you go back to the drawing board - literally. DRAW your intended circuit on paper before you write any VHDL. VHDL is a hardware description language. Not a programming language. Writing code like software will not work.