import bpy
import os

def make_transition(seq, s1, s2):
    trans = seq.sequences.new_effect("wipe", 'WIPE', 4, 1, 0, s1, s2)
    trans.angle = 90

def create_slide_show(scn, dirname):

    if scn.sequence_editor is None:
        scn.sequence_editor_create()

    fr0 = 1
    nframes=90
    overlap = 30
    i=0
    prevStrip = None
    for basename in sorted(os.listdir(dirname)):
        if 0 == i%2:
            ch = 2
        else:
            ch = 3
        strip = scn.sequence_editor.sequences.\
            new_image(name=basename,  filepath=os.path.join(dirname, basename), channel=ch, frame_start=fr0)
        strip.frame_final_duration = nframes+ overlap

        if prevStrip is not None:
            make_transition(scn.sequence_editor, prevStrip, strip)

        prevStrip = strip
        fr0 = fr0 + nframes
        i=i+1



create_slide_show(bpy.context.scene, "/var/tmp/blender/frog-hop")