You can sort photos in a gallery in two ways:
By capture time (the images must have a capture time on the file itself)
By upload date and time (default).
For each of the options, you can select to sort ascending or descending.
To sort photos in a gallery, click the "Sort" button in the header of the "Photos" panel while editing a gallery, and choose from the options available.
Why are my images out of order when I sort alphabetically?
Sprout acts like most photo browsers and file managers when sorting alphabetically - it sorts by the first number it finds in the filename. Therefore, if you name your files like this:
Image-1, Image-2, Image-3 ... Image-10, Image-11, etc
Then Sprout will sort them with Image-1 first, and then Image-10, because it sees the 1 in 10 first, and 1 comes before the 2 in Image-2.
This is why we recommend naming your files with a zero-padding, so that your filenames will look more like this:
Image-001, Image-002, Image-003 ... Image-010, Image-011, etc
When you name your files this way, Sprout (and every other photo browser and file manager) will sort them as you'd expect because it sees the zeros and can put them in the proper order. Make sure you have the proper amount of zeros in comparison to the amount of images you're exporting. For example, if you have 1000 images, you'll need your images to have 3 zeros before the filename (Image-0001).
How to rename files properly in Lightroom
To rename files with zero-padding (as described above) in Lightroom:
Select all of your photos.
Click Library > Rename Photos.
From the "File Naming" dropdown, click the "Edit" option.
Remove any text in the editor.
Insert "Custom Text" from the bottom "Custom" section.
Type in a dash (to separate your custom text from the sequence).
Insert "Sequence # (0001)" from the "Sequence and Date" section.
Save this as a preset. Now, when you rename files, you'll be able to type in a custom text (i.e. the client's name) and then it'll automatically zero-pad your numbers.