The amount of image data you can fit in (I assume) 256 bytes is pretty small. That's enough for an uncompressed 8-bit 16x16 pixel bitmap with no header. And you

The amount of image data you can fit in (I assume) 256 bytes is pretty small. That's enough for an uncompressed 8-bit 16x16 pixel bitmap with no header. And you only get 3/4 of that amount of information if it's base64'd. With compression you can do better, but 256 bytes is not going to get you anywhere near the kind of high-resolution artwork that people seem to be using NFTs for.