The basic formula is widthxheightx4, because regardless of the type of image (jpg,png,etc) they are converted to a bitmap in RAM. Depending on how old the GPU is, a power of 2 calculation could also be be needed before calculating RAM usage.
Image is 2000x900 PNG
RAM use would be approx. 2000x900x4 = 7.2MB
If power of 2 applies then it would be: 2048x1024x4 = 8.4MB