Use Big, High-resolution Photos
This goes for every print product. But it is especially important for larger print formats, such as wall calendars. It is very common to place a photo across the entire calendar page, so you have to make sure the photo quality is good.
Hiring a photographer to create professional photos for your custom photo calendar is always a good idea. If that does not fit your budget, there are many stock photography websites.
Pay Attention to Safe Zones
In printing, a safe zone is an area of your artwork that will not be affected by cutting, hole drilling, binding, or any other finishing process. The majority of your design will be within the safe zone. The end of a safe zone is usually somewhere near the edges of the paper, but it does depend on what the printed product is.
All essential design elements should never go over the safe zone. This includes logos, text, or important parts of an image.
Choose a design style that you like and stick with it throughout the entire calendar. This goes for photos, as well as for the calendar pages themselves. Don't switch up fonts and change the calendar layout for every month.
Calendars should be pretty straightforward and not too complicated. When it comes to photos, you can have images of your products or of people using your products. In case you don't have photos that are this specific, pick a theme and stick with it.