TARAN was tested using PNG and JPEG file formats -- these two are "officially" supported. In practice, TARAN should understand any image format that your browser can handle.
What's more important is the picture itself:
1. Individual shots must be visible; one ragged hole = not ok. Therefore, at most 3 shots per group at 100m / 100 yards.
2. There should be a known distance measure on target to set the scale (e.g. grid, or simply a line of known length drawn with a ruler).
3. In order to produce meaningful results, the target image should be free from geometrical distortions -- a flatbed scanner is perfect.
A photo is ok under two conditions: (a) narrow angle (big zoom from a few steps away) to avoid barrel/pincushion distortion, and (b) photo lens axis perpendicular to the target, aimed at the centre, to avoid perspective-related distortion. In other words, if using a photo, your target rectangle should be rectangle in the photo -- straight lines, right angles.
Example -- such image is not good:
(the target angles are not right, the sides are not parallel)
Example -- such image is ok:
(the target angles are quite right, the sides are more or less parallel)
4. Of course, no sights/zero adjustment between series (otherwise averaging different series is meaningless).