The picture is blocky/quantized/pixelated or all black
Terms in your expression that should be evaluated in floating point are being interpreted as integers. x and y are both integers, so to force something to be evaluated in floating point you can either use a (float) cast, or, if your expression has constants, include a decimal point.
The picture is in colour but it's supposed to be greyscale.
You have used one of the strings "red", "green", "blue", "white" somewhere. Don't multiply by white to make a greyscale pictures. Just use values.
The picture is supposed to be in colour, but not those colours.
When a colour is multiplied by a negative value, that colour is taken away! (Try setting the picture to -1*green, for example.) It's usually simplest to keep your colours positive.
The picture is just like the last one I made, not what it should be.
Your C expression got past the compiler but created a runtime error. The most likely possibility is that somewhere in your picture there is a divide-by-zero. E.g. something like y/x won't work because down the middle of the picture, x is zero.
I get a text picture that grumbles about me using a string that isn't allowed. But I want to use that string.
Sorry. If the facility would be really useful, you could email me to request it.