When an image is in the public domain, you can use it freely without attribution. When an image has been licensed under a free license, you can use it and give proper attribution. Both of these cases are not very likely for most comics.
It is actually against copyright laws to take whatever you want and use it but there is something called Fair Use in the USA which should apply because HubPages is hosted in that country. This means that you can use content within reasonable limits and if you give proper attribution. I see many hubs that simply use any image they want but you can get into trouble if you do that - because the content is not legally yours.
I try to avoid this by using only free images. I even avoid fair use of images because there are many free images available anyway. That is not the case for your comic but it is up to you if you want to use the image under those conditions.