Side-view is essential: almost nothing can be done if the hair or costume is not shown exactly from the side (i.e. 90°)
Anime-style is essential: it means two or three colours (base, shadow, light reflection) with no or very simple gradients. Photographs, 3D game screenshots, digital "waterpainting" pictures, etc. have colour gradients very difficult to reproduce.
High resolution is essential: very pixelated refs makes drawing small parts quite difficult to reproduce (find PNG or JPG images, not BMP).
Upload your references on an offsite image hoster (e.g. Imgur) and put them in image tags