There's a new feature that was added to the Doximity Fax feature for iOS users regarding image quality.
When sending any image in a fax, tap on the 'Toggle' button located to the right of 'Contains Images' on the bottom of the display to enable this feature.
When you enable this feature, it will retain the image quality.
Please note: this feature will have to be turned on each time you send a fax that contains images, as it is not automatically enabled. This feature will also be coming soon for Android.