An experimental study was performed on spray characteristics of spray diesel (D100) and biodiesel blend (BD65) injected into an atmospheric chamber. A qualitative analysis of spray images was conducted through exploiting the image processing with common image processing software. The results showed that the posterization of the images offered more detailed qualitative information on the spray compared to the more commonly-used threshold method. The posterized images showed the existence of layers in the spray with its transition at different grey levels. At lower injection, the spray tip penetration of BD65 was slightly lower than D100, whereas at high injection pressure, spray tip penetration of BD65 was higher than D100. Although BD65 had lower maximum velocity, the higher density of biodiesel may have resulted in greater momentum that enabled BD65 to have longer spray tip penetration at higher injection pressure. At higher injection pressure, the spray angle of BD65 tended to be less than that of D100.