Your best bet for a good reference will always be a reference sheet. But sometimes these aren't easily available. In that case, you can collect images that meet these criteria:

• Find art (Either 2D, 3D, or even a screenshot) that gets a full, unobstructed view of a character's front. In most cases, a view of the back and a view of the side are not necessary. However, if a character has a unique detail on their side or back (Such as a pattern), then finding reference images for these would be helpful. If you want them drawn at that angle, finding a reference image is required.
• Find art that has a clear shot of the character's face. One should be able to easily distinguish features when zooming in on the image, and should not be met with a mess of pixels.
• Larger images are better! They make distinguishing details much easier.
• The images do not need to be compiled into a single image. Feel free to send multiple images!

• Neutral lighting is important! Neutral lighting ensures that a character is colored correctly.
• Make sure the character's color palette is correct when you submit your reference.

Below is an example of a good set of images to use as a reference. Since the top image is missing information (What type of shoes is she wearing?) the bottom two images are used to supplemant that information.
REMEMBER: You do not need to compile all the images on the same canvas, you can send different images.