Where are these images beng transferred from that you need to use DropBox or email?
Are these images on a Windows PC? Directly from a digital camera?
What is the orginal format of these images?
Are these images been imported into a computer (PC or Mac)?
You can sync images using iTunes on a PC, instead of using DropBox or email.
There is more than one way to get images onto an iPad.
There are apps that can transfer images wirelessly over either WiFi or Bluetooth between iDevices or an iDevice and a computer.
There is some reason why DropBox is transferring a clean, clear image while it is failing to do so via email.