My suggestion is to use Gimp, either manually as described below, or using the script Divide Scanned Images1 2 for automatic detection.
As it gives me better control, I would open the scanned image, and repeat following:
- Mark with rectangle a given receipt
- Copy, Ctrl+c (or Cut, Ctrl+x)
- Use Edit > Paste As > New image, Shift+Ctrl+v 3
After copying/cutting all receipts, I would close original image, and save each receipt with a proper name.
A variation of this is to do the following:
- Duplicate image, Ctrl+d
- Mark rectangle with receipt
- Crop to selection, Shift+C
- Save, Ctrl+s
- Close window, Alt+F4 (returning to original image)
Note that the link provided from SuperUser, lists this example, the "Divide Scanned Images" script, and some other methods, including one using ImageMagick which looks kind of interesting if you are into command line scripting.
1 Can the Gimp split an image into multiple images? from SuperUser SE
3 Little uncertain on shortcuts as I use a Mac when writing this answer