I am not sure what might be causing this. When emailing, you are always up against the limitation of the size of the email (typically 20 megabytes). However emailing 1 or 2 images would not reach this limit. One bug in the app I am aware of is the Black & White option when exporting images. This causes the images to grow quite big (rather than what you would expect it do, that is shrink the photo). Please be sure the Color option is selected. When you go into the expense entries and tap the Photo Receipt field, are you able to preview the image? If you can that would insure the image is safely stored on your device and should most definitely be able to be exported.
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