Well, it kinda depends on what images you're using, y'know? =P I mean, /roll is kinda designed to display dice images. There are a number of game systems that don't use dice with normal numbered faces, and rollable tables are an answer to that. If you just want to put a larger image in the chat, you can use [alt text](image url) and you'll get the image in chat, automatically scaled to the width of the chat. If you want a randomly selected image in the chat that's also large... well, I think your only option is the API, which requires a Pro subscription.