Don't have different shades of white!
I also had a habit of displaying images without any descriptions/context. I've put a simple template under this series from the Savage project, these are just drafts and the font could be a lot better, but it gives an idea of one way to keep the images looking consistent and contextualised.
For these character variations she's used a gradiented grey background to suggest three-dimensional space, which really makes the character feel more grounded and 'real'.
This is a realistic environment concept she made for a games company. She used a simple, black bar across the bottom with a neat, clean font. There are no distracting borders or anything - all of the focus is the artwork itself.
I'm considering ordering some books and generally brushing up on graphic design, I feel like a crucial part of being a digital artist is understanding subtle colour shifts, resolutions, font, layout, etc. yet I've never really had the opportunity to learn about it in detail. It would also be very useful to understand how to prepare images for print.
At this point it is good to remind myself about this post last year I made about Chris Oatley's portfolio advice. Read the page again! Need to remember to put this into practice.